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This article is the second of a two-part series about understanding and managing pH. “Understanding pH in Reef Tanks: Part One” discusses what pH is, why it might be worth increasing, and what influences pH in reef tanks. In this part, we’ll go over the exact causes of low pH and exactly what to do to finally increase it. There will also be some pH-related problem solving and interesting bits of information I’ve gathered over the years.
Table of contents:
Figure 1. May 2024. My starting photo for the “Points and Says ‘Wow Nice Efflo’” acropora grow-out contest.
Causes of Low pH
If you haven’t already read the first part, the most important thing to understand is that low pH issues are caused by elevated carbon dioxide and low alkalinity in aquariums. Both of these factors have the most substantial impact on the resulting pH, and the most effective pH-boosting solutions ultimately rely on altering one or both of these factors.
Carbon dioxide can enter the aquarium in two ways: it can be produced directly within the system through certain maintenance methods or it can dissolve into the tank from excessively elevated indoor CO₂ levels. On the other hand, very low alkalinity reduces the water’s ability to buffer or resist against CO₂ and acidic compounds, causing deeper pH dips. Let’s go through the factors that cause low pH issues in reef tanks.
Elevated Indoor Carbon Dioxide:
Elevated indoor carbon dioxide will readily dissolve into the aquarium, and it’s a very common culprit in many low pH scenarios. CO₂ can accumulate when windows and doors remain shut for prolonged periods. Most sources of excessive indoor carbon dioxide come from people and pets respiring. In addition, gas appliances like stoves, ovens, fireplaces, and other fuel-burning sources release carbon dioxide as byproducts, increasing the home’s CO₂ concentration if not properly vented. It’s common to see pH drop during large family gatherings, and many reefers report their highest pH readings when the home is vacant for extended times, like when traveling.
Low Alkalinity:
By definition, alkalinity is the water’s buffering capacity and resistance to acidic compounds. “Understanding pH in Reef Tanks: Part One” shows a simple graph of how the resulting pH is determined by the carbon dioxide and alkalinity levels. Most aquariums do best with alkalinity between 7–11 dKH, and lower values will result in an overall lower pH with less resistance to acidity and carbon dioxide.
Carbon Dosing to Reduce Nitrate:
All organic carbon dosing will ultimately reduce the pH when added to an aquarium because carbon dioxide is released during bacterial metabolism. Vodka releases its acidity gradually as the organic is consumed, but vinegar front-loads 50% instantly as H⁺ upon addition and releases the remaining like vodka. Despite vinegar releasing H⁺, the acetate later consumed will counteract and bring the alkalinity back to its starting point. There is no permanent reduction. When dosing equal carbon molecules, vinegar and vodka release the same total acidity, but to reiterate, any organic carbon dosing will release CO₂ and reduce pH once it is metabolized.
Sulfur Denitrators:
Another way to reduce nitrate in the aquarium is through the use of sulfur denitrators. Reactors filled with elemental sulfur achieve by allowing denitrifying bacteria to convert nitrate into nitrogen gas (N₂) that is effectively off-gassed via aeration. This process also results in H⁺, but unlike dosing vinegar, it will permanently reduce some alkalinity because there is no acetate to counteract it. The alkalinity loss can be calculated: every 50 ppm nitrate removed through a sulfur denitrator produces enough acid (H⁺) to simultaneously drop 2.3 dKH alkalinity, and it will need to be replaced by more dosing (Holmes-Farley, 2015). It only depletes alkalinity, not calcium or magnesium.
Alkalinity Additives:
Certain alkalinity additives can cause a slight, temporary decrease in pH when added to an aquarium. Baking soda and organic carbon–based additives such as All-For-Reef by Tropic Marin and All in One by Seachem have this slight pH-decreasing effect. Any reduction of pH from these additives is minor, and many successful tanks use them. However, if raising the pH is the primary goal, there are much better alkalinity additives.
Calcium Reactors:
Calcium reactors hold crushed aragonite in a reaction chamber, and a CO₂ gas tank lowers the pH enough to redissolve the aragonite into bioavailable calcium and bicarbonate ions. However, the resulting solution can contain a lot of carbon dioxide that is then released into the tank. It’s common for aquariums running these reactors to have their pH on the lower end of the spectrum (Holmes-Farley, 2016).
Ammonia:
The conversion of ammonia into nitrate temporarily depletes alkalinity and lowers pH. Fortunately, the depleted alkalinity completely returns once the resulting nitrate is consumed (Holmes-Farley, 2016). This net-zero alkalinity effect goes for all ammonia additions, whether from feeding or dosing. The only exception is when adding ammonia from ammonium chloride. Upon addition, it dissociates into ammonia and hydrochloric acid (HCl), which permanently lowers alkalinity and pH in the same way and ratio as sulfur denitrators. Additional alkalinity dosing will be required if using ammonium chloride. This is why ammonium bicarbonate is used and preferred
Too Many Fish:
This cause is fairly straightforward. Fish are always respirating and releasing carbon dioxide through their gills, and very high fish stocking densities can result in high imports of carbon dioxide in the tank. The amount of CO₂ produced by a fish community depends on how much food is added, and heavily stocked tanks naturally require more food. However, I would not consider reducing the fish population as a first-line approach to solving low pH issues unless the tank is severely overstocked and causing a poor quality of life for the inhabitants. I’m mentioning this here to show how carbon dioxide can enter a tank in ways most people don’t even consider.
Low Gas Exchange:
If gas exchange from powerheads and skimmers is insufficient, it becomes very difficult to export the excess carbon dioxide produced in the tank, leading to a buildup that results in low pH. The aeration test in Understanding pH in Reef Tanks: Part One will determine if low gas exchange is the culprit and whether additional aeration can effectively solve the pH issue.
Methods that increase pH
The desire to maintain higher pH usually stems from one primary goal: increasing corals' growth rate. Each week, countless threads are made by reefers around the world trying to figure out why their pH is stuck at 7.8, and in many cases, even lower. There are many different ways we can raise the pH, and the tanks that successfully achieve high pH, whether intentional or not, use the methods we’re about to cover.
Algae-Based Filtration:
Photosynthesis consumes a significant amount of carbon dioxide, and it’s the reason pH is usually much higher during the day. Refugiums and algae scrubbers take advantage of this by growing dense algae in a controlled space, away from herbivores, and allows for easy harvest. Algae require light, so you can decide when this pH-boosting effect occurs. Many reefers run their refugiums at night to raise pH and oxygen when both are naturally at their lowest. This also raises the overnight baseline, which reduces the day-to-night pH swing and indirectly increases the daytime pH, too. For example, it’s much easier to finish the photoperiod at pH 8.3 when starting from 8.1 rather than 7.8.
High pH Alkalinity Additives:
There are three additives that not only supply alkalinity but also significantly boost pH as a side effect. These are sodium carbonate (soda ash), sodium hydroxide (lye), and calcium hydroxide (kalkwasser). Soda ash is simply baked sodium bicarbonate (baking soda), and it’s a good starting point. However, hydroxide-based additives provide twice the pH boost when raising or maintaining equal units of alkalinity. Any pH-boosting additive works by binding H⁺ or CO₂ from the water upon addition, converting it into bicarbonate. While kalkwasser and sodium hydroxide are the highest pH-boosting additives available for reef tanks, but they are caustic and have a small learning curve to implement safely.
Increase the Alkalinity Demand:
It can be difficult to raise the pH if there isn’t much calcification or alkalinity demand in the tank. When dosing high-pH additives, a greater demand allows for more dosing, which results in a bigger pH boost. Additionally, raising the alkalinity target toward the upper end of the recommended range (9–11 dKH) naturally helps maintain higher pH even under the same CO₂ exposure.
Bring in Fresh Air:
If weather and air quality permit, opening windows can bring in low-CO₂ air and help vent indoor buildup. In many cases, simply opening windows around the home can lead to a noticeable pH increase within a day or two.
Fresh Air to Skimmer:
Protein skimmers normally draw air from inside the house, but in high indoor CO₂ environments, this can keep the pH suppressed by injecting excess carbon dioxide into the water. If there’s a nearby window, you can run a tube from the skimmer pump to draw in fresh outdoor air (which is only around 400 ppm CO₂). It is effective at raising pH. This method requires very little maintenance or upkeep and is considered one of the first-line approaches for increasing pH. Rather than leaving a window cracked open, some reefers drill a small hole through the wall for the intake tube, though doing so safely is beyond the scope of this article. Make sure the intake tube isn’t placed in an area where it could draw in pesticides. Also, consider adding a mesh guard and activated carbon at the input to prevent debris and airborne pollutants from entering the tank, respectively.
CO₂ Scrubbers:
CO₂ scrubber use a canister filled with soda lime, which is a pelletized mixture of calcium hydroxide and sodium hydroxide. Drawing the skimmer’s intake through this media completely removes CO₂ from the air before it enters the tank, making this method highly effective at raising pH, and even better than drawing in outdoor air. While CO₂ scrubbers are highly effective, they require regular media replacement, which adds both maintenance and recurring cost. The media cannot be regenerated in standard ovens and requires extreme heat (around 700 °C) to reuse (Rogalewicz et al., 2020). For this reason, it’s often wise to start feeding a protein skimmer with outside air and only switch to a CO₂ scrubber if the pH boost isn’t adequate.
Recirculating Scrubbers (Should You Do It?):
Some people recirculate their CO₂ scrubbers. It essentially draws air to the scrubber from the skimmer collection cup rather than from the ambient indoor air. This can save money because the air inside the cup is already partially depleted of CO₂ when a scrubber is in use, so it seems logical and efficient to recirculate it rather than constantly expose the media to CO₂-rich indoor air.
However, there are two main drawbacks to recirculating scrubbers. First, turning a skimmer into a closed loop eliminates one of its greatest benefits: providing oxygenation and gas exchange. It’s essentially recycling the same air. Almost nothing is more effective at gas exchange than a protein skimmer, and losing this benefit is a suboptimal tradeoff.
The second concern is that if the skimmer cup ever overflowed with water, the intake tube could pull water through the scrubber and release a highly concentrated hydroxide solution back into the tank. This can substantially spike pH and alkalinity, even leading to a tank crash. Several failsafes are required when using recirculating scrubbers to help prevent this from occurring as best as possible.
I used to recirculate my scrubber to save media, but I realized oxygen may play a bigger role in coral growth than we give it credit for. Wijgerde et al. (2014) found that Acropora millepora’s calcification rates significantly increased with higher pH during the day, but at night, it was oxygen saturation that limited calcification. Raising pH from 7.8 to 8.1 did not meaningfully increase growth at night under low O₂ conditions. When oxygen saturation was raised to 100%, both pH 7.8 and 8.1 showed significantly higher growth, nearly reaching the same calcification rate. This suggests oxygen might be even more important than pH for growth, at least at night. If you purchased a CO₂ scrubber to increase growth but fully recirculate it, you might actually be reducing growth during a substantial portion of the corals’ growth window.
There are two good workarounds to save media while preserving the skimmer’s role in gas exchange. The first is to run a partial recirculating setup, where the skimmer lid isn’t completely sealed or part of the scrubber’s intake still pulls in room air. The second option, and likely the best all-around, is to draw natural outside air through the scrubber. This saves media by starting with lower CO₂ levels, ensures there is fresh oxygen and proper gas exchange, and eliminates the failure risk if the skimmer cup ever overflowed.
Figure 2. May 2025. Exactly one year after the starting photo. My efflo colony (pictured here) was what the judges used to determine the 1st-place grow-out winner. My pH was consistently around 8.3.
How to Increase pH for Coral Growth: My Recipe for Success
I realize many people just want a clear guideline they can follow to reliably raise their pH. Here’s my exact recipe that I personally use to maintain pH 8.3–8.4 in my aquarium. I’m very confident this method will work for almost any tank dealing with low pH problems.
Signs of Excessively High pH
It’s very difficult to reach a pH that is too high. Most tanks can run as high as 8.5–8.6 with no issues. In a normal home environment with typical CO₂ levels, using a basic alkalinity additive like All-For-Reef alongside standard reefkeeping practices makes it unlikely to see pH consistently over 8.3. However, if you stack on most of the pH-raising methods and eliminate the factors that usually keep the it suppressed, it is possible to run into a high-pH problem.
The biggest risk of excessively elevated pH is precipitation. Calcium carbonate is already supersaturated in seawater (Chave & Suess, 1970), and pushing the pH too high can induce a precipitation event. This wastes additives and makes it very difficult to maintain proper calcium and alkalinity levels in the aquarium.
Signs of Abnormally High pH Include:
How to Stop Precipitation Events
When a precipitation event occurs, a large number of fresh calcium carbonate surfaces are created. These act as seed crystals, encouraging further precipitation and creating a snowball effect that can be difficult to stop. Seawater stays supersaturated with calcium carbonate because several natural factors normally inhibit precipitation. To break the cycle, enough time without producing new seed crystals must pass for these inhibitors to “coat” the bare CaCO₃ surfaces.
If excessive precipitation is present, follow this regimen for one to two weeks before slowly reintroducing any pH-boosting methods. Take care not to let pH or alkalinity rise as high as before, at least not initially:
How to Reduce pH (and Alkalinity) in Emergencies
The pH can spike to dangerously high levels during alkalinity overdoses, but there are several ways to safely bring it back down. First, a pH of 8.6 or less does not require drastic intervention. It is generally safe for corals and will naturally come down on its own. When taking steps to actively reduce pH, take frequent measurements throughout the process so you don’t accidentally overshoot and end up with the opposite problem of dangerously low pH.
Increase Gas Exchange (Lowers pH):
Increasing gas exchange is the simplest approach. Aeration with indoor air will naturally push carbon dioxide into the tank and gradually lower pH. After all, excess indoor CO₂ is one of the main causes of low pH in reef tanks. Point powerheads toward the surface to agitate the waterline as much as possible. The more surface ripples, the better. Protein skimmers work very well here because they directly inject indoor air into the aquarium. If the pH is especially high, I would remove the skimmer cup and let the water overflow into the sump to maximize contact with ambient air. In any case, increasing gas exchange is the least invasive option.
Add CO₂ Directly (Lowers pH):
Another low-risk way to lower pH is by adding CO₂ directly. Plain, unflavored seltzer or carbonated water is simply water with dissolved carbon dioxide, and it can bring pH down more quickly than aeration alone.
Should You Reduce Alkalinity?
If a pH spike was caused by an alkalinity overdose, you’re likely dealing with two issues: elevated pH and high alkalinity. If alkalinity is below 11 dKH, there usually isn’t a need to intervene. Natural coral consumption and abiotic precipitation will gradually bring it back down, as long as all alkalinity dosing is stopped. However, some acropora and other sensitive corals may not tolerate prolonged exposure to very high alkalinity, and in those cases, taking action quickly can be less harmful than leaving them in those conditions.
Dosing Vinegar to Reduce pH
Some people recommend dosing vinegar to lower pH and alkalinity, but there are two problems with this approach. First, any alkalinity reduced by vinegar will return once bacteria metabolize the organic carbon. Second, dosing large amounts of vinegar to lower pH can lead to bacterial blooms. I do not recommend considering vinegar dosing for solving high pH and alkalinity.
Mineral Acids (if pH and Alkalinity are too high):
Mineral acids permanently reduce alkalinity and pH. These acids will significantly lower pH and generate a large amount of CO₂ that must be off-gassed. Muriatic acid (commonly sold at hardware stores) and sodium bisulfate are two options for lowering alkalinity. It’s not recommended to directly dose mineral acids directly into a running reef tank because it will likely cause the pH to drop low. If you go extremely slowly, you can reduce the alkalinity over time in a reef tank, but you must go slow and take frequent pH and alkalinity measurements to prevent overshooting and causing the opposite problem. I would only consider dosing mineral acids if the pH is also too elevated in addition to alkalinity.
Water Change (Lowers Alkalinity Under One Condition):
If the pH is already on the lower end, you can’t safely reduce alkalinity directly in the tank without driving the pH too low. Water changes, in and of themselves, are highly ineffective at reducing pH and alkalinity unless you change a significant amount of water. However, you can overcome this limitation by first reducing the alkalinity of the water-change reservoir prior to use. Many reefers use mineral acids to drop the alkalinity of new saltwater to match the tank’s level. Before use, the water must be heavily aerated for 24 hours to expel the excess CO₂ generated during the alkalinity reduction process. If the tank’s alkalinity is too high, you can reduce the alkalinity of the mixing container to even as low as 0–1 dKH, but be sure to aerate thoroughly for a full day before performing the water change.
Common pH Misconceptions
For the final topic, I’d like to go over some common pH-related misconceptions and maybe even share a few pro tips I’ve learned over the years.
There is no such thing as a liquid pH-boosting solution that doesn’t also increase alkalinity.
pH-boosting effects from liquid solutions are temporary. Carbon dioxide will eventually re-enter the tank and bring the pH back to baseline once dosing stops.
Do not dose pH additives purely to maintain pH because the alkalinity will eventually climb too high.
Only partially using pH-boosting additives for alkalinity demand (for example, dosing All-For-Reef during the day and filling the remaining gaps at night with high-pH additives) will not yield the same results as letting pH-boosting additives handle the entire alkalinity demand. If the goal is achieving the best possible pH gains, it’s better to fully switch over to carbonate or hydroxide additives.
Using sodium carbonate or sodium hydroxide to manage pH without dosing the corresponding calcium part will eventually depress calcium, magnesium, and other major ions. Sodium-based alkalinity additives do not supply balanced calcium. Calcium hydroxide (kalkwasser) doesn’t have this concern because it contains both calcium and alkalinity in the single solution.
Sodium bicarbonate mixed with sodium hydroxide makes sodium carbonate.
Mixing two pH solutions will not give a pH between them because pH is logarithmic.
Calcium hydroxide saturated in vinegar will never reach the same pH as calcium hydroxide saturated in water. The vinegar neutralizes some hydroxide by forming calcium acetate, which makes calcium the limiting factor during saturation. In a test I did, fully saturated calcium hydroxide in water reached pH 12.5, while fully saturating it in vinegar reached pH 12.1.
Most soft corals will not grow faster at higher pH or alkalinity because they do not contain calcium carbonate. There are a few soft corals that contain calcium carbonate spicules, but those species aren’t very common.
Low phosphate and organic content are why new saltwater tends to precipitate calcium carbonate if left circulating for too long. This is why we can keep water circulating in a reef tank indefinitely, but not in a water-change container without eventually causing precipitation.
If the alkalinity of your salt mix is too low, only use sodium bicarbonate (baking soda) to raise it. There is no better additive for this purpose. Using high-pH additives will only cause precipitation and have little to no effect on the aquarium’s pH
Heavily aerating new saltwater after reducing the alkalinity will bring the pH back up to nearly its original level. However, when lowering the alkalinity of a salt mix to near 0 dKH (for the purpose of reducing high alkalinity in an aquarium), aeration will only raise the pH to the low 7s due to the almost nonexistent buffering capacity. Despite this low pH, it will not significantly lower the aquarium’s pH after aeration, for the same reason that freshwater top-offs don’t drop pH.
Adding pure freshwater (at pH 7) to an aquarium containing 35 ppt salinity and pH 8.1 will cause the pH to increase.
Figure 3. September 2025. Four months after winning, the growth continues strong. I might have to start selling frags soon.
Conclusion
If you’ve made it this far, thank you. I was inspired to write this two-part series after seeing so many threads of people struggling with pH issues. I originally intended to fit everything into one article, but I couldn’t stop writing. pH is one of the most confusing parameters in reefkeeping, and there’s a ton of misinformation about it. I hope this series has brought some clarity and maybe even helped some of you finally achieve your target pH. Whether to “chase pH” is a personal decision, but I believe everyone should have the knowledge and tools to safely raise their levels if they choose.
References:
Chave, K. E., & Suess, E. (1970). Calcium carbonate saturation in seawater: Effects of dissolved organic matter. Limnology and Oceanography, 15(4), 633–637. https://doi.org/10.4319/lo.1970.15.4.0633
Holmes-Farley, R. (2015). Nitrate in the reef aquarium. Reef Edition. http://www.reefedition.com/nitrate-in-the-reef-aquarium/
Holmes-Farley, R. (2016). pH and the reef aquarium. Reef2Reef. https://www.reef2reef.com/ams/ph-and-the-reef-aquarium.7/
Rogalewicz, B., Czylkowska, A., Anielak, P., & Samulkiewicz, P. (2020). Investigation and possibilities of reuse of carbon dioxide absorbent used in anesthesiology. Materials, 13(21), 5052. https://doi.org/10.3390/ma13215052
Wijgerde, T., Silva, C. I. F., Scherders, V., van Bleijswijk, J., & Osinga, R. (2014). Coral calcification under daily oxygen saturation and pH dynamics reveals the important role of oxygen. Biology Open, 3(6), 489–493. https://doi.org/10.1242/bio.20147922
Table of contents:
- Causes of Low pH
- Methods that Increase pH
- How to Increase pH for Coral Growth: My Recipe for Success
- Signs of Excessively High pH
- How to Stop Precipitation Events
- How to Reduce pH (and Alkalinity) in Emergencies
- Common pH Misconceptions
- Conclusion
Figure 1. May 2024. My starting photo for the “Points and Says ‘Wow Nice Efflo’” acropora grow-out contest.
Causes of Low pH
If you haven’t already read the first part, the most important thing to understand is that low pH issues are caused by elevated carbon dioxide and low alkalinity in aquariums. Both of these factors have the most substantial impact on the resulting pH, and the most effective pH-boosting solutions ultimately rely on altering one or both of these factors.
Carbon dioxide can enter the aquarium in two ways: it can be produced directly within the system through certain maintenance methods or it can dissolve into the tank from excessively elevated indoor CO₂ levels. On the other hand, very low alkalinity reduces the water’s ability to buffer or resist against CO₂ and acidic compounds, causing deeper pH dips. Let’s go through the factors that cause low pH issues in reef tanks.
Elevated Indoor Carbon Dioxide:
Elevated indoor carbon dioxide will readily dissolve into the aquarium, and it’s a very common culprit in many low pH scenarios. CO₂ can accumulate when windows and doors remain shut for prolonged periods. Most sources of excessive indoor carbon dioxide come from people and pets respiring. In addition, gas appliances like stoves, ovens, fireplaces, and other fuel-burning sources release carbon dioxide as byproducts, increasing the home’s CO₂ concentration if not properly vented. It’s common to see pH drop during large family gatherings, and many reefers report their highest pH readings when the home is vacant for extended times, like when traveling.
Low Alkalinity:
By definition, alkalinity is the water’s buffering capacity and resistance to acidic compounds. “Understanding pH in Reef Tanks: Part One” shows a simple graph of how the resulting pH is determined by the carbon dioxide and alkalinity levels. Most aquariums do best with alkalinity between 7–11 dKH, and lower values will result in an overall lower pH with less resistance to acidity and carbon dioxide.
Carbon Dosing to Reduce Nitrate:
All organic carbon dosing will ultimately reduce the pH when added to an aquarium because carbon dioxide is released during bacterial metabolism. Vodka releases its acidity gradually as the organic is consumed, but vinegar front-loads 50% instantly as H⁺ upon addition and releases the remaining like vodka. Despite vinegar releasing H⁺, the acetate later consumed will counteract and bring the alkalinity back to its starting point. There is no permanent reduction. When dosing equal carbon molecules, vinegar and vodka release the same total acidity, but to reiterate, any organic carbon dosing will release CO₂ and reduce pH once it is metabolized.
Sulfur Denitrators:
Another way to reduce nitrate in the aquarium is through the use of sulfur denitrators. Reactors filled with elemental sulfur achieve by allowing denitrifying bacteria to convert nitrate into nitrogen gas (N₂) that is effectively off-gassed via aeration. This process also results in H⁺, but unlike dosing vinegar, it will permanently reduce some alkalinity because there is no acetate to counteract it. The alkalinity loss can be calculated: every 50 ppm nitrate removed through a sulfur denitrator produces enough acid (H⁺) to simultaneously drop 2.3 dKH alkalinity, and it will need to be replaced by more dosing (Holmes-Farley, 2015). It only depletes alkalinity, not calcium or magnesium.
Alkalinity Additives:
Certain alkalinity additives can cause a slight, temporary decrease in pH when added to an aquarium. Baking soda and organic carbon–based additives such as All-For-Reef by Tropic Marin and All in One by Seachem have this slight pH-decreasing effect. Any reduction of pH from these additives is minor, and many successful tanks use them. However, if raising the pH is the primary goal, there are much better alkalinity additives.
Calcium Reactors:
Calcium reactors hold crushed aragonite in a reaction chamber, and a CO₂ gas tank lowers the pH enough to redissolve the aragonite into bioavailable calcium and bicarbonate ions. However, the resulting solution can contain a lot of carbon dioxide that is then released into the tank. It’s common for aquariums running these reactors to have their pH on the lower end of the spectrum (Holmes-Farley, 2016).
Ammonia:
The conversion of ammonia into nitrate temporarily depletes alkalinity and lowers pH. Fortunately, the depleted alkalinity completely returns once the resulting nitrate is consumed (Holmes-Farley, 2016). This net-zero alkalinity effect goes for all ammonia additions, whether from feeding or dosing. The only exception is when adding ammonia from ammonium chloride. Upon addition, it dissociates into ammonia and hydrochloric acid (HCl), which permanently lowers alkalinity and pH in the same way and ratio as sulfur denitrators. Additional alkalinity dosing will be required if using ammonium chloride. This is why ammonium bicarbonate is used and preferred
Too Many Fish:
This cause is fairly straightforward. Fish are always respirating and releasing carbon dioxide through their gills, and very high fish stocking densities can result in high imports of carbon dioxide in the tank. The amount of CO₂ produced by a fish community depends on how much food is added, and heavily stocked tanks naturally require more food. However, I would not consider reducing the fish population as a first-line approach to solving low pH issues unless the tank is severely overstocked and causing a poor quality of life for the inhabitants. I’m mentioning this here to show how carbon dioxide can enter a tank in ways most people don’t even consider.
Low Gas Exchange:
If gas exchange from powerheads and skimmers is insufficient, it becomes very difficult to export the excess carbon dioxide produced in the tank, leading to a buildup that results in low pH. The aeration test in Understanding pH in Reef Tanks: Part One will determine if low gas exchange is the culprit and whether additional aeration can effectively solve the pH issue.
Methods that increase pH
The desire to maintain higher pH usually stems from one primary goal: increasing corals' growth rate. Each week, countless threads are made by reefers around the world trying to figure out why their pH is stuck at 7.8, and in many cases, even lower. There are many different ways we can raise the pH, and the tanks that successfully achieve high pH, whether intentional or not, use the methods we’re about to cover.
Algae-Based Filtration:
Photosynthesis consumes a significant amount of carbon dioxide, and it’s the reason pH is usually much higher during the day. Refugiums and algae scrubbers take advantage of this by growing dense algae in a controlled space, away from herbivores, and allows for easy harvest. Algae require light, so you can decide when this pH-boosting effect occurs. Many reefers run their refugiums at night to raise pH and oxygen when both are naturally at their lowest. This also raises the overnight baseline, which reduces the day-to-night pH swing and indirectly increases the daytime pH, too. For example, it’s much easier to finish the photoperiod at pH 8.3 when starting from 8.1 rather than 7.8.
High pH Alkalinity Additives:
There are three additives that not only supply alkalinity but also significantly boost pH as a side effect. These are sodium carbonate (soda ash), sodium hydroxide (lye), and calcium hydroxide (kalkwasser). Soda ash is simply baked sodium bicarbonate (baking soda), and it’s a good starting point. However, hydroxide-based additives provide twice the pH boost when raising or maintaining equal units of alkalinity. Any pH-boosting additive works by binding H⁺ or CO₂ from the water upon addition, converting it into bicarbonate. While kalkwasser and sodium hydroxide are the highest pH-boosting additives available for reef tanks, but they are caustic and have a small learning curve to implement safely.
Increase the Alkalinity Demand:
It can be difficult to raise the pH if there isn’t much calcification or alkalinity demand in the tank. When dosing high-pH additives, a greater demand allows for more dosing, which results in a bigger pH boost. Additionally, raising the alkalinity target toward the upper end of the recommended range (9–11 dKH) naturally helps maintain higher pH even under the same CO₂ exposure.
Bring in Fresh Air:
If weather and air quality permit, opening windows can bring in low-CO₂ air and help vent indoor buildup. In many cases, simply opening windows around the home can lead to a noticeable pH increase within a day or two.
Fresh Air to Skimmer:
Protein skimmers normally draw air from inside the house, but in high indoor CO₂ environments, this can keep the pH suppressed by injecting excess carbon dioxide into the water. If there’s a nearby window, you can run a tube from the skimmer pump to draw in fresh outdoor air (which is only around 400 ppm CO₂). It is effective at raising pH. This method requires very little maintenance or upkeep and is considered one of the first-line approaches for increasing pH. Rather than leaving a window cracked open, some reefers drill a small hole through the wall for the intake tube, though doing so safely is beyond the scope of this article. Make sure the intake tube isn’t placed in an area where it could draw in pesticides. Also, consider adding a mesh guard and activated carbon at the input to prevent debris and airborne pollutants from entering the tank, respectively.
CO₂ Scrubbers:
CO₂ scrubber use a canister filled with soda lime, which is a pelletized mixture of calcium hydroxide and sodium hydroxide. Drawing the skimmer’s intake through this media completely removes CO₂ from the air before it enters the tank, making this method highly effective at raising pH, and even better than drawing in outdoor air. While CO₂ scrubbers are highly effective, they require regular media replacement, which adds both maintenance and recurring cost. The media cannot be regenerated in standard ovens and requires extreme heat (around 700 °C) to reuse (Rogalewicz et al., 2020). For this reason, it’s often wise to start feeding a protein skimmer with outside air and only switch to a CO₂ scrubber if the pH boost isn’t adequate.
Recirculating Scrubbers (Should You Do It?):
Some people recirculate their CO₂ scrubbers. It essentially draws air to the scrubber from the skimmer collection cup rather than from the ambient indoor air. This can save money because the air inside the cup is already partially depleted of CO₂ when a scrubber is in use, so it seems logical and efficient to recirculate it rather than constantly expose the media to CO₂-rich indoor air.
However, there are two main drawbacks to recirculating scrubbers. First, turning a skimmer into a closed loop eliminates one of its greatest benefits: providing oxygenation and gas exchange. It’s essentially recycling the same air. Almost nothing is more effective at gas exchange than a protein skimmer, and losing this benefit is a suboptimal tradeoff.
The second concern is that if the skimmer cup ever overflowed with water, the intake tube could pull water through the scrubber and release a highly concentrated hydroxide solution back into the tank. This can substantially spike pH and alkalinity, even leading to a tank crash. Several failsafes are required when using recirculating scrubbers to help prevent this from occurring as best as possible.
I used to recirculate my scrubber to save media, but I realized oxygen may play a bigger role in coral growth than we give it credit for. Wijgerde et al. (2014) found that Acropora millepora’s calcification rates significantly increased with higher pH during the day, but at night, it was oxygen saturation that limited calcification. Raising pH from 7.8 to 8.1 did not meaningfully increase growth at night under low O₂ conditions. When oxygen saturation was raised to 100%, both pH 7.8 and 8.1 showed significantly higher growth, nearly reaching the same calcification rate. This suggests oxygen might be even more important than pH for growth, at least at night. If you purchased a CO₂ scrubber to increase growth but fully recirculate it, you might actually be reducing growth during a substantial portion of the corals’ growth window.
There are two good workarounds to save media while preserving the skimmer’s role in gas exchange. The first is to run a partial recirculating setup, where the skimmer lid isn’t completely sealed or part of the scrubber’s intake still pulls in room air. The second option, and likely the best all-around, is to draw natural outside air through the scrubber. This saves media by starting with lower CO₂ levels, ensures there is fresh oxygen and proper gas exchange, and eliminates the failure risk if the skimmer cup ever overflowed.
Figure 2. May 2025. Exactly one year after the starting photo. My efflo colony (pictured here) was what the judges used to determine the 1st-place grow-out winner. My pH was consistently around 8.3.
How to Increase pH for Coral Growth: My Recipe for Success
I realize many people just want a clear guideline they can follow to reliably raise their pH. Here’s my exact recipe that I personally use to maintain pH 8.3–8.4 in my aquarium. I’m very confident this method will work for almost any tank dealing with low pH problems.
- Maintain alkalinity around 9–10 dKH.
- Use only kalkwasser or sodium hydroxide to supply alkalinity.
- Keep enough corals and coralline algae to maintain a strong alkalinity demand.
- Feed your skimmer with outside air or run it through a CO₂ scrubber (scrubbers are more effective but require media replacement).
- Review the “Causes of Low pH” section and reduce or eliminate as many of those factors as possible.
Signs of Excessively High pH
It’s very difficult to reach a pH that is too high. Most tanks can run as high as 8.5–8.6 with no issues. In a normal home environment with typical CO₂ levels, using a basic alkalinity additive like All-For-Reef alongside standard reefkeeping practices makes it unlikely to see pH consistently over 8.3. However, if you stack on most of the pH-raising methods and eliminate the factors that usually keep the it suppressed, it is possible to run into a high-pH problem.
The biggest risk of excessively elevated pH is precipitation. Calcium carbonate is already supersaturated in seawater (Chave & Suess, 1970), and pushing the pH too high can induce a precipitation event. This wastes additives and makes it very difficult to maintain proper calcium and alkalinity levels in the aquarium.
Signs of Abnormally High pH Include:
- Unusually high alkalinity consumption without enough coral or coralline algae growth to justify it, and testing errors have been ruled out
- A “snow globe” effect, with white flakes that don’t clear
- Water that appears hazy
- Heaters, pumps, or UV sterilizers developing sand-like deposits, white crust, or even start seizing
- UV quartz sleeves requiring cleaning much more frequently than usual
- Sump chambers developing a frosted, opaque coating and feel rough to the touch
- Heavy crust buildup around alkalinity dosing lines, sometimes caking on in severe cases
- Sand bed hardening into chunks, resembling clumping cat litter
- Media bags, filter rollers, or filter socks becoming stiff
- GFO media or cartridges showing excessive white deposits
How to Stop Precipitation Events
When a precipitation event occurs, a large number of fresh calcium carbonate surfaces are created. These act as seed crystals, encouraging further precipitation and creating a snowball effect that can be difficult to stop. Seawater stays supersaturated with calcium carbonate because several natural factors normally inhibit precipitation. To break the cycle, enough time without producing new seed crystals must pass for these inhibitors to “coat” the bare CaCO₃ surfaces.
If excessive precipitation is present, follow this regimen for one to two weeks before slowly reintroducing any pH-boosting methods. Take care not to let pH or alkalinity rise as high as before, at least not initially:
- Limit efforts to raise pH.
- Keep alkalinity on the lower end (6.5–7 dKH).
- Use low-pH alkalinity additives: sodium bicarbonate is inexpensive, pure, and widely available. All-For-Reef is also a good choice
- Avoid dosing calcium carbonate (coral snow), as it simply adds more bare calcium carbonate surfaces.
- Keep phosphate at or above 0.10 ppm.
- Maintain magnesium at 1,300 ppm or higher.
- Increase organics by feeding a bit heavier.
- (Optional) Carbon dose: it can slightly lower pH and encourages bacterial growth that helps coat bare calcium carbonate surfaces.
How to Reduce pH (and Alkalinity) in Emergencies
The pH can spike to dangerously high levels during alkalinity overdoses, but there are several ways to safely bring it back down. First, a pH of 8.6 or less does not require drastic intervention. It is generally safe for corals and will naturally come down on its own. When taking steps to actively reduce pH, take frequent measurements throughout the process so you don’t accidentally overshoot and end up with the opposite problem of dangerously low pH.
Increase Gas Exchange (Lowers pH):
Increasing gas exchange is the simplest approach. Aeration with indoor air will naturally push carbon dioxide into the tank and gradually lower pH. After all, excess indoor CO₂ is one of the main causes of low pH in reef tanks. Point powerheads toward the surface to agitate the waterline as much as possible. The more surface ripples, the better. Protein skimmers work very well here because they directly inject indoor air into the aquarium. If the pH is especially high, I would remove the skimmer cup and let the water overflow into the sump to maximize contact with ambient air. In any case, increasing gas exchange is the least invasive option.
Add CO₂ Directly (Lowers pH):
Another low-risk way to lower pH is by adding CO₂ directly. Plain, unflavored seltzer or carbonated water is simply water with dissolved carbon dioxide, and it can bring pH down more quickly than aeration alone.
Should You Reduce Alkalinity?
If a pH spike was caused by an alkalinity overdose, you’re likely dealing with two issues: elevated pH and high alkalinity. If alkalinity is below 11 dKH, there usually isn’t a need to intervene. Natural coral consumption and abiotic precipitation will gradually bring it back down, as long as all alkalinity dosing is stopped. However, some acropora and other sensitive corals may not tolerate prolonged exposure to very high alkalinity, and in those cases, taking action quickly can be less harmful than leaving them in those conditions.
Dosing Vinegar to Reduce pH
Some people recommend dosing vinegar to lower pH and alkalinity, but there are two problems with this approach. First, any alkalinity reduced by vinegar will return once bacteria metabolize the organic carbon. Second, dosing large amounts of vinegar to lower pH can lead to bacterial blooms. I do not recommend considering vinegar dosing for solving high pH and alkalinity.
Mineral Acids (if pH and Alkalinity are too high):
Mineral acids permanently reduce alkalinity and pH. These acids will significantly lower pH and generate a large amount of CO₂ that must be off-gassed. Muriatic acid (commonly sold at hardware stores) and sodium bisulfate are two options for lowering alkalinity. It’s not recommended to directly dose mineral acids directly into a running reef tank because it will likely cause the pH to drop low. If you go extremely slowly, you can reduce the alkalinity over time in a reef tank, but you must go slow and take frequent pH and alkalinity measurements to prevent overshooting and causing the opposite problem. I would only consider dosing mineral acids if the pH is also too elevated in addition to alkalinity.
Water Change (Lowers Alkalinity Under One Condition):
If the pH is already on the lower end, you can’t safely reduce alkalinity directly in the tank without driving the pH too low. Water changes, in and of themselves, are highly ineffective at reducing pH and alkalinity unless you change a significant amount of water. However, you can overcome this limitation by first reducing the alkalinity of the water-change reservoir prior to use. Many reefers use mineral acids to drop the alkalinity of new saltwater to match the tank’s level. Before use, the water must be heavily aerated for 24 hours to expel the excess CO₂ generated during the alkalinity reduction process. If the tank’s alkalinity is too high, you can reduce the alkalinity of the mixing container to even as low as 0–1 dKH, but be sure to aerate thoroughly for a full day before performing the water change.
Common pH Misconceptions
For the final topic, I’d like to go over some common pH-related misconceptions and maybe even share a few pro tips I’ve learned over the years.
There is no such thing as a liquid pH-boosting solution that doesn’t also increase alkalinity.
pH-boosting effects from liquid solutions are temporary. Carbon dioxide will eventually re-enter the tank and bring the pH back to baseline once dosing stops.
Do not dose pH additives purely to maintain pH because the alkalinity will eventually climb too high.
Only partially using pH-boosting additives for alkalinity demand (for example, dosing All-For-Reef during the day and filling the remaining gaps at night with high-pH additives) will not yield the same results as letting pH-boosting additives handle the entire alkalinity demand. If the goal is achieving the best possible pH gains, it’s better to fully switch over to carbonate or hydroxide additives.
Using sodium carbonate or sodium hydroxide to manage pH without dosing the corresponding calcium part will eventually depress calcium, magnesium, and other major ions. Sodium-based alkalinity additives do not supply balanced calcium. Calcium hydroxide (kalkwasser) doesn’t have this concern because it contains both calcium and alkalinity in the single solution.
Sodium bicarbonate mixed with sodium hydroxide makes sodium carbonate.
Mixing two pH solutions will not give a pH between them because pH is logarithmic.
Calcium hydroxide saturated in vinegar will never reach the same pH as calcium hydroxide saturated in water. The vinegar neutralizes some hydroxide by forming calcium acetate, which makes calcium the limiting factor during saturation. In a test I did, fully saturated calcium hydroxide in water reached pH 12.5, while fully saturating it in vinegar reached pH 12.1.
Most soft corals will not grow faster at higher pH or alkalinity because they do not contain calcium carbonate. There are a few soft corals that contain calcium carbonate spicules, but those species aren’t very common.
Low phosphate and organic content are why new saltwater tends to precipitate calcium carbonate if left circulating for too long. This is why we can keep water circulating in a reef tank indefinitely, but not in a water-change container without eventually causing precipitation.
If the alkalinity of your salt mix is too low, only use sodium bicarbonate (baking soda) to raise it. There is no better additive for this purpose. Using high-pH additives will only cause precipitation and have little to no effect on the aquarium’s pH
Heavily aerating new saltwater after reducing the alkalinity will bring the pH back up to nearly its original level. However, when lowering the alkalinity of a salt mix to near 0 dKH (for the purpose of reducing high alkalinity in an aquarium), aeration will only raise the pH to the low 7s due to the almost nonexistent buffering capacity. Despite this low pH, it will not significantly lower the aquarium’s pH after aeration, for the same reason that freshwater top-offs don’t drop pH.
Adding pure freshwater (at pH 7) to an aquarium containing 35 ppt salinity and pH 8.1 will cause the pH to increase.
Figure 3. September 2025. Four months after winning, the growth continues strong. I might have to start selling frags soon.
Conclusion
If you’ve made it this far, thank you. I was inspired to write this two-part series after seeing so many threads of people struggling with pH issues. I originally intended to fit everything into one article, but I couldn’t stop writing. pH is one of the most confusing parameters in reefkeeping, and there’s a ton of misinformation about it. I hope this series has brought some clarity and maybe even helped some of you finally achieve your target pH. Whether to “chase pH” is a personal decision, but I believe everyone should have the knowledge and tools to safely raise their levels if they choose.
References:
Chave, K. E., & Suess, E. (1970). Calcium carbonate saturation in seawater: Effects of dissolved organic matter. Limnology and Oceanography, 15(4), 633–637. https://doi.org/10.4319/lo.1970.15.4.0633
Holmes-Farley, R. (2015). Nitrate in the reef aquarium. Reef Edition. http://www.reefedition.com/nitrate-in-the-reef-aquarium/
Holmes-Farley, R. (2016). pH and the reef aquarium. Reef2Reef. https://www.reef2reef.com/ams/ph-and-the-reef-aquarium.7/
Rogalewicz, B., Czylkowska, A., Anielak, P., & Samulkiewicz, P. (2020). Investigation and possibilities of reuse of carbon dioxide absorbent used in anesthesiology. Materials, 13(21), 5052. https://doi.org/10.3390/ma13215052
Wijgerde, T., Silva, C. I. F., Scherders, V., van Bleijswijk, J., & Osinga, R. (2014). Coral calcification under daily oxygen saturation and pH dynamics reveals the important role of oxygen. Biology Open, 3(6), 489–493. https://doi.org/10.1242/bio.20147922
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