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Few parameters in reefkeeping cause as much disagreement as pH. Some consider managing it, let alone testing it, unnecessary or even potentially harmful. While pH usually falls into an acceptable range in most tanks, it can be borderline low in others, causing stress and the risk of dissolving coral skeletons. Although some might have their pH in the safe, lower end, they might desire higher values to increase coral growth. In any case, for those who struggle to reach their pH target, this simple parameter can become a significant source of frustration.
pH measures how acidic or basic a solution is based on the concentration of hydrogen ions [H⁺] in the water. The scale ranges from 0 to 14, with 7 as neutral. Values below 7 are acidic, while values above 7 are basic (alkaline). The pH scale is logarithmic: a one-point drop (pH 8.0 to 7.0) means the water is 10 times more acidic, but a two-point drop (8.0 to 6.0) is 100 times more acidic. A seemingly insignificant 0.1 unit decrease, such as from pH 8.0 to 7.9, increases acidity by 26%. In reef tanks, pH plays a major role in calcification, which is when corals and other calcifying creatures deposit calcium carbonate to form hard structures like skeletons or shells. pH also strongly affects abiotic precipitation, where calcium and carbonate ions combine and "wastes" calcium and alkalinity from the water. An example is when heaters or pumps develop a hard, white crust. An increase of 0.3 pH units increases the risk of precipitation as much as doubling the total alkalinity or calcium level. Alternatively, pH values below 7.7 can be low enough to start dissolving calcium carbonate, potentially putting coral skeletons and other calcium-based structures at risk (Holmes-Farley, 2016).
This article is the first of a two-part series about understanding and managing pH. Part one will explain what pH is, what causes it to reach the levels experienced by reefers, how to determine if the reading is even accurate, and how to diagnose where the low-pH issue is coming from.
Part two will discuss how to target specific pH levels, how to fix, manage, and prevent pH-related problems, and will also debunk common misconceptions that can leave people spinning in circles. The goal is to leave you fully equipped to achieve any desired pH level and to better troubleshoot pH-related problems.
Part Two can be found here:
https://www.reef2reef.com/threads/how-to-raise-ph-in-reef-tanks.1129406/
Table of Contents:
The Two Factors that Affect pH in Aquariums
pH has an inverse relationship with the hydrogen (H⁺) concentration in the water, meaning that as the H⁺ in the solution increases, the pH decreases or becomes more acidic. As you remove H⁺, the pH increases. There are two primary factors that play a major role in determining the concentration of H⁺, and it's manipulating these factors that can control the resulting pH in reef tanks.
The first factor is carbon dioxide. CO₂ has the strongest impact on the pH of seawater. We know that only excess hydrogen ions (H⁺) can cause the pH to drop, but carbon dioxide doesn’t contain a single hydrogen ion, so how can it lower pH? When CO₂ enters the aquarium, it binds with water (H₂O) to form a weak acid called carbonic acid (H₂CO₃), which isn’t very stable in seawater. It doesn't directly drop the pH. However, carbonic acid quickly breaks apart (dissociates) into bicarbonate (HCO₃⁻) and a hydrogen ion (H⁺), and it’s that released hydrogen ion that causes the pH to drop. At first glance, it might seem like this process increases alkalinity by adding bicarbonate. However, every time you gain a bicarbonate ion, you also gain a hydrogen ion, and they effectively cancel each other out. Normal alkalinity sources supply bicarbonate or carbonate without the extra H⁺ released from carbonic acid, giving the aquarium a net increase in alkalinity.
The second factor is alkalinity. By definition, alkalinity is the amount of acid required to lower the pH of a solution to a certain level. To put it simply, it’s the water’s resistance to acidic compounds, otherwise known as the aquarium’s buffering capacity. There are several compounds that contribute to alkalinity, but the ones we care about are bicarbonate and carbonate, since those are what help corals grow and function optimally. Directly measuring the concentration of bicarbonate and carbonate is very challenging. Fortunately, the total alkalinity is predominantly composed of those ions, and it is very simple to test (Holmes-Farley, 2002). Alkalinity has a large effect on the resulting pH. When an aquarium is perfectly aerated with ambient CO₂, the resulting pH is determined entirely by the alkalinity (Holmes-Farley, 2016). Higher alkalinity levels have more resistance to pH changes and drops than lower levels (Holmes-Farley, 2004).
This graph, created by @rishma, visually represents the relationship between carbon dioxide, alkalinity, and pH.
The Interaction Between Alkalinity and CO₂
Indoor CO₂ levels in homes typically range between 500 and 1,000 ppm, which is considered safe for people. However, high indoor carbon dioxide can significantly lower the pH in seawater. In my own home, I’ve seen CO₂ spike to 900–1,000 ppm, and even higher during large family gatherings and additional cooking. For example, at 700 ppm CO₂ with alkalinity of 6.5 dKH (natural seawater), the aquarium’s pH after full CO₂
equilibrium would be about 7.9. Raising the alkalinity to 10 dKH at the same CO₂ level keeps the pH around 8.1. Outdoor air is usually around 400 ppm CO₂; at that level, 6 dKH maintains a pH near 8.1, while 11 dKH keeps it at around pH 8.4.
Do You Need to Raise Your pH?
The most commonly accepted pH range for reef tanks is between 7.8 and 8.6. Most aquariums will naturally fall within this range and maintain proper coral health as long as the alkalinity is within 7–11 dKH and calcium is at least 400 ppm (Holmes-Farley, 2004). Many hobbyists worry when they see a pH reading around 7.8, fearing it will harm their corals. I tried searching for a clear answer on the 'best' pH level, but even the research studies seem mixed.
Atkinson et al. (1995) maintained and grew 57 different stony corals long term at the Waikiki Aquarium in Hawaii. The incoming water ranged from pH 7.5 to 7.8, and the aquarium itself maintained a pH range of 7.6 to 8.3. Despite frequent exposure to the very low end of the scale, the corals’ growth rates were similar to the upper rates reported from the field.
Interestingly, just because some corals thrive at a certain pH doesn’t mean others will. Some species can resist acidified water by raising their internal pH at the calcification site (Venn et al., 2013), while others lack this ability or do so inefficiently (McCulloch et al., 2012). In fact, some corals may even grow better at lower pH levels (Anthony et al., 2008; Tanvet et al., 2023), while others may suffer slower growth and increase their risk of bleaching (Anthony et al., 2008).
Coralline algae, in particular, seem to be especially vulnerable to reduced pH. (Cornwall et al., 2022; Kuffner et al., 2008; Ragazzola et al., 2012). Overall, studies seem to show that higher pH is generally better for coral growth. For example, daytime calcification in acropora millepora increased by 178% at pH 8.4 compared to pH 8.1 under normal oxygen conditions (Wijgerde et al., 2014). Additionally, stylophora pistillata grew best at pH 8.0–8.2 compared to pH 7.6, especially when the alkalinity was higher (Marubini et al., 2008). Comeau et al. (2015) found that reef communities exposed to elevated CO₂ (~1300 ppm) for eight weeks showed a 29% decrease in calcification compared to those at natural outdoor CO₂ levels (~400 ppm).
In most cases, the motivation to raise pH comes from a single goal: faster growth. If you’re filling out a new tank and want to reach maturity sooner, have a coral farm and need to speed up production, or even want an extra edge in grow-out contests, chasing growth from every angle may be desirable.
Unfortunately, ocean and lab tank studies don’t always translate to the same results we might experience in home aquariums. An example is how most reef tanks would starve and risk dinoflagellates if they emulated the inorganic nutrient levels of the ocean. While lab tanks and home aquariums both keep corals in four-cornered, glass boxes, experimental tanks usually haven’t reached the same level of maturity and husbandry that our setups contain. Possibly most inconvenient, most studies don’t test the corals commonly kept in the hobby. As we’ve seen above, even closely related species can respond very differently to the same pH conditions. Finding studies that test the corals we keep was like searching for a needle in a haystack.
Anecdotally, many hobbyists report noticeably improved coral growth after increasing the pH, and the studies generally seem to agree that higher pH can provide better growth compared to lower levels, but determining how much more growth it will influence, as we’ve seen, is tricky. Many hobbyists, especially those with mature and overgrown corals, find increasing their pH to be extra work with no space to sustain the extra growth. In those cases, it may actually work in their favor to allow the pH to settle around the lower end. Others, however, choose to go the extra mile, aiming to maximize growth wherever possible. Some believe higher pH decreases a coral's vulnerability to STN, though the evidence for that isn’t clear. Rather than debate the “ideal” number, this article will focus on understanding and managing pH from a clear, practical, and unbiased perspective.
How Elevated pH Might Make Calcification Easier
There are two main mechanisms a higher pH might improve growth. Coral skeletons are composed of calcium carbonate (CaCO₃), which requires carbonate ions as the primary building blocks. In seawater, carbonate is already in short supply compared to bicarbonate, and as pH decreases, more carbonate ions are converted into bicarbonate, further reducing their availability for calcification (Smithsonian Institution, n.d.). When carbonate is in short supply, many corals can internally convert bicarbonate into carbonate, but this process results in an extra hydrogen ion (H⁺) that the calcifying organism must expel to complete calcification. When the surrounding water already contains a high concentration of H⁺, it becomes more difficult to pump out the excess hydrogen against the gradient, and this limitation might even outweigh carbonate unavailability (Jokiel, 2013).
Challenges with Measuring pH
Before deciding whether to increase the pH, you’ll first need to know what it is. If there’s one takeaway from this article, it’s that pH is one of the most difficult parameters to measure accurately. pH naturally fluctuates throughout the day and night, and even within the hour. Photosynthesis is the primary reason why the pH trends upwards as the day progresses, reaching its peak by the very end of the photoperiod. At night, when photosynthesis stops, the pH decreases, hitting its lowest reading prior to the lights turning back on. It’s relatively common to see a 0.3 pH swing throughout a 24-hour period, so a single pH reading at a moment in time will not result in an accurate depiction of the entire pH cycle.
The pH of calibration standards isn't stable, either. While proper standards are buffered to resist shifting from carbon dioxide, they’re not fully resistant, especially the pH 10 solutions. Additionally, not all calibration standards are made accurately to begin with, and determining whether a calibration solution is adequate is often based on trust. pH testers can only be as accurate as the standards they’re calibrated with. It’s a paradox: you need an accurate buffer to calibrate your probe, but how can you determine the buffer’s accuracy if you don’t already have a calibrated probe?
Finally, pH probes can drift over time. I personally notice my probe significantly shifts after 1–2 months, but many may begin to drift even sooner. The only way to determine if the probe has lost its accuracy is to test it against standards or recalibrate it and compare the old and new values.
Choosing a pH Probe
The pH of a reef tank is always subject to change, especially between day and night, which is why I prefer inline probes that monitor the pH 24/7. I have a probe connected to my controller system, and it logs data to a graph in real time, allowing me to visualize trends as recent as the past hour up to a full week. It’s worth knowing the highs and lows of the pH cycle prior to deciding whether pH-boosting efforts are warranted.
A good probe should support two-point calibrations using pH 7 and 10 standards. Using two reference points achieves better accuracy than if the probe can only extrapolate the voltage from a one point. For the type of probe, I prefer lab-grade, double-junction probes. Unlike single-junction probes, these prevent precipitation on the porous junction by keeping the silver chloride solution away from direct contact with seawater. They tend to last longer and hold calibration better than the single-junction types.
Additionally, I prefer probes with a ±0.01 pH accuracy. This level of resolution helps determine whether pH adjustment efforts are moving in the right direction, as changes can initially be subtle. It can feel tedious if the probe isn’t accurate enough to display progress, and sometimes having better accuracy can determine whether a method is starting to make a measurable difference versus having no effect at all. Finally, although a minor detail, plastic or epoxy-bodied probes are more durable and less likely to crack during handling and calibration compared to glass-bodied ones.
That being said, I like to keep things practical and realistic. As long as you calibrate regularly and check against reliable standards, even a basic pH probe can be useful. The safe range for reef tanks is fairly broad, so high precision mainly matters when you’re actively trying to maintain a specific target. The only thing I’d avoid using is pH test kits or strips. Holmes-Farley (2016) notes that test kits relying on color indicator dyes aren’t the best way to measure pH in reef aquariums. They can be inaccurate, difficult to interpret, may not cover the ideal range, degrade over time, and can’t be verified against a known standard. In my observations, the most extreme cases of low or high readings almost always stem from dye-based test kits, so I’d avoid them entirely.
Choosing pH Calibration Standards
pH buffers used for two-point calibrations should bracket the tested range. The optimal pH for reef tanks is between 7.8 and 8.6, which is why pH 7 and 10 buffers are recommended for calibration. pH 4 and 7 buffer solutions are available, but those are better suited for acidic environments, such as in calcium reactors. Calibration solutions come in several types of packaging: bottles, single-use foil packets, and even tablets one can dissolve in water at home. Individually sealed packets ensure each buffer is fresh, with no air exposure prior to use, and that’s what I use. Bottled pH solutions can work, as long as they are closed immediately after use and the probe is not inserted directly into the bottle. Opening and closing a bottle, or forgetting to recap it, can allow CO₂ to enter, altering its stated pH. In addition to air exposure, temperature alone can change pH, which is why most buffers include a reference chart showing the optimal temperature during calibration. Lastly, any pH solution, even when stored correctly, will eventually degrade as CO₂ slowly seeps in. While a very slow process, even individually sealed single-use packets aren't immune to this, and that’s why most quality buffers include an expiration date. Using inaccurate calibration solutions will result in an inaccurate probe. To put it in perspective, I’ve personally run some calculations showing a couple of scenarios:
If the pH 7 solution is actually 7.4 and the pH 10 solution is accurate, a true pH of 8.0 would appear as 7.7 (false low).
If the pH 7 solution is 7.5 and the pH 10 solution is 10.5, a true pH of 8.0 would read as 7.5 (false low)
If the pH 7 solution is accurate and the pH 10 solution is actually 9.5, a true 8.0 would read as 8.2 (false high).
If the pH 7 is actually 6.5 and the pH 10 solution is actually 9.5, a true pH of 8.0 would appear as 8.5 (false high).
Since pH 10 solutions are more likely to be lower, rather than above, their stated level, it appears that false highs might be more probable from inaccurate pH 10 calibration standards. Fortunately, many of the available commercial buffers seem to be accurate, at least they were when Holmes-Farley (2005) tested them. They were all within ±0.1 of the set standard. In no particular order, the companies whose buffers passed were Hanna, Milwaukee, Thermo Fisher, and Pinpoint. Of course, that test was done decades ago, but I personally feel comfortable using any of the above brands. Other commercial brands purchased online may work, as well.
How to Calibrate a pH Probe
Calibrating a pH probe is relatively simple, but there are slight variations depending on the type of probe being used. Some models allow you to input specific calibration values, for example, pH 7.54 instead of a flat pH 7.0. In those cases, you can initially calibrate their probes with normal 7 and 10 pH calibration solutions. Afterwards, they have the ability to test the pH of older calibration bottles lying around, mark their exact pH readings, and then use those specific pH solutions for future calibrations. However, most common probes require calibrating with exactly pH 7.0 and 10.0, with no option to deviate. This tutorial is written with those probes in mind.
To run the calibration, you’ll need a pH probe (and controller, if applicable), pH 7.0 and 10.0 calibration solutions around their optimally stated temperature, a cup of RO/DI or distilled water, and scissors, if you're using individually sealed packets.
The Aeration Test
Low pH issues in aquariums are almost always caused by excess CO₂, and there are two possible sources: the aquarium itself or elevated indoor carbon dioxide. An inexpensive CO₂ meter can determine if the ambient air is the culprit, but it won’t readily tell whether the aquarium itself is also contributing to the problem. Fortunately, there’s a straightforward, low-cost diagnostic you can run at home in just a couple of hours that can identify the exact source, and it's called The Aeration Test (Holmes-Farley 2004). The test singles out the source by aerating the 'aquarium' in different conditions and measuring the pH's response to them. Below is my summary of the method.
To run the test, you’ll need an 8 oz cup of tank water, a calibrated pH probe, a timer, an air pump with an air stone, and access to the outdoors. Only use a small cup because off-gassing carbon dioxide is very time-consuming. A small sample ensures the water will be fully equilibrated after the set time for accurate results.
1) If heavily aerating a cup indoors increases the pH, there is either not enough aeration or some filtration/maintenance routine is adding too much CO₂ or H⁺.
2) If aerating the cup indoors doesn’t meaningfully raise the pH, but aerating it outdoors does, the culprit is excess indoor carbon dioxide entering the tank. The solution is to increase the pH-boosting methods ('Managing pH in Reef Tanks: Part Two' will discuss how).
3) If both the indoor and outdoor cup pH values remain low after the test, or if the outdoor sample rises, but not meaningfully, then the probe, test kit, calibration, or buffer standards are very likely inaccurate. The outdoor aeration part of the test determines if the probe is accurately responding to low CO₂ conditions and shows how concentrated the indoor CO₂ is.
Conclusion
Understanding pH in Reef Tanks: Part one primes us for the next section. In order to know how to effectively control pH, you'd first need to understand how pH works and why it behaves the way it does. Once you see it, it becomes crystal clear what makes a pH-boosting method effective, and you can even determine which methods fall short. In the next part, we'll go over all of the potential causes of low pH issues, how to eliminate them, and I'll highlight exactly how to raise pH and never allow it to be low again, if desired. We'll also go over some precipitation problem-solving. By the time you finish the second part, you might be struggling with pH that is too high.
References
Anthony, K. R. N., Kline, D. I., Diaz-Pulido, G., Dove, S., & Hoegh-Guldberg, O. (2008). Ocean acidification causes bleaching and productivity loss in coral reef builders. Proceedings of the National Academy of Sciences, 105(45), 17442–17446. https://doi.org/10.1073/pnas.0804478105
Atkinson, M. J., Carlson, B. A., & Crow, G. L. (1995). Coral growth in high-nutrient, low-pH seawater: A case study of corals cultured at the Waikiki Aquarium, Honolulu, Hawaii. Coral Reefs, 14(4), 215–223. https://doi.org/10.1007/BF00334344
Comeau, S., Carpenter, R. C., Lantz, C. A., & Edmunds, P. J. (2015). Ocean acidification accelerates dissolution of experimental coral reef communities. Biogeosciences, 12(2), 365–372. https://doi.org/10.5194/bg-12-365-2015
Cornwall, C. E., Diaz-Pulido, G., Comeau, S., & McCulloch, M. T. (2022). Understanding coralline algal responses to ocean acidification: Meta‐analysis and synthesis. Global Change Biology, 28(2), 362–374. https://doi.org/10.1111/gcb.15899
Holmes-Farley, R. (2002). Chemistry and the aquarium: What is alkalinity? reefs.com.https://reefs.com/magazine/chemistry-and-the-aquarium-what-is-alkalinity/
Holmes-Farley, R. (2004, September). Low pH: Causes and cures. Reefkeeping Magazine.http://reefkeeping.com/issues/2004-09/rhf/index.htm
Holmes-Farley, R. (2005, February). A comparison of pH calibration buffers. Reefkeeping Magazine.http://reefkeeping.com/issues/2005-02/rhf/index.htm
Holmes-Farley, R. (2016). pH and the reef aquarium. Reef2Reef. https://www.reef2reef.com/ams/ph-and-the-reef-aquarium.7/
Jokiel, P. L. (2013). Coral reef calcification: Carbonate, bicarbonate and proton flux under conditions of increasing ocean acidification. Proceedings of the Royal Society B: Biological Sciences, 280(1764), 20130031. https://doi.org/10.1098/rspb.2013.0031
Kuffner, I. B., Andersson, A. J., Jokiel, P. L., Rodgers, K. S., & Mackenzie, F. T. (2008). Decreased abundance of crustose coralline algae due to ocean acidification. Nature Geoscience, 1(2), 114–117. https://doi.org/10.1038/ngeo100
Marubini, F., Ferrier-Pagès, C., Furla, P., & Allemand, D. (2008). Coral calcification responds to seawater acidification: A working hypothesis towards a physiological mechanism. Coral Reefs, 27(3), 491–499. https://doi.org/10.1007/s00338-008-0375-6
McCulloch, M., Falter, J., Trotter, J., & Montagna, P. (2012). Coral resilience to ocean acidification and global warming through pH up-regulation. Nature Climate Change, 2(8), 623–627. https://doi.org/10.1038/nclimate1473
Ragazzola, F., Foster, L. C., Form, A. U., Anderson, P. S. L., Hansteen, T. H., & Fietzke, J. (2012). Ocean acidification weakens the structural integrity of coralline algae. Global Change Biology, 18(9), 2804–2812. https://doi.org/10.1111/j.1365-2486.2012.02756.x
Smithsonian Institution. (n.d.). Ocean acidification. Smithsonian Ocean. https://ocean.si.edu/ocean-life/invertebrates/ocean-acidification
Tanvet, C., Allemand, D., Dufresne, A., Pouget-Cuvelier, A., Vidal-Dupiol, J., Gilson, E., Houlbrèque, F., & Reynaud, S. (2023). Corals adapted to extreme and fluctuating seawater pH increase calcification rates and have unique symbiont communities. Ecology and Evolution, 13(5), e10099. https://doi.org/10.1002/ece3.10099
Venn, A. A., Tambutté, E., Holcomb, M., Laurent, J., Allemand, D., & Tambutté, S. (2013). Impact of seawater acidification on pH at the tissue–skeleton interface and calcification in reef corals. Proceedings of the National Academy of Sciences, 110(5), 1634–1639. https://doi.org/10.1073/pnas.1216153110
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
pH measures how acidic or basic a solution is based on the concentration of hydrogen ions [H⁺] in the water. The scale ranges from 0 to 14, with 7 as neutral. Values below 7 are acidic, while values above 7 are basic (alkaline). The pH scale is logarithmic: a one-point drop (pH 8.0 to 7.0) means the water is 10 times more acidic, but a two-point drop (8.0 to 6.0) is 100 times more acidic. A seemingly insignificant 0.1 unit decrease, such as from pH 8.0 to 7.9, increases acidity by 26%. In reef tanks, pH plays a major role in calcification, which is when corals and other calcifying creatures deposit calcium carbonate to form hard structures like skeletons or shells. pH also strongly affects abiotic precipitation, where calcium and carbonate ions combine and "wastes" calcium and alkalinity from the water. An example is when heaters or pumps develop a hard, white crust. An increase of 0.3 pH units increases the risk of precipitation as much as doubling the total alkalinity or calcium level. Alternatively, pH values below 7.7 can be low enough to start dissolving calcium carbonate, potentially putting coral skeletons and other calcium-based structures at risk (Holmes-Farley, 2016).
This article is the first of a two-part series about understanding and managing pH. Part one will explain what pH is, what causes it to reach the levels experienced by reefers, how to determine if the reading is even accurate, and how to diagnose where the low-pH issue is coming from.
Part two will discuss how to target specific pH levels, how to fix, manage, and prevent pH-related problems, and will also debunk common misconceptions that can leave people spinning in circles. The goal is to leave you fully equipped to achieve any desired pH level and to better troubleshoot pH-related problems.
Part Two can be found here:
https://www.reef2reef.com/threads/how-to-raise-ph-in-reef-tanks.1129406/
Table of Contents:
- The Two Factors that Affect pH in Aquariums
- The Interaction Between Alkalinity and CO₂
- Do You Need to Raise Your pH?
- How Elevated pH Might Make Calcification Easier
- Challenges with Measuring pH
- Choosing a pH Probe
- Choosing pH Calibration Standards
- How to Calibrate a pH Probe
- The Aeration Test
The Two Factors that Affect pH in Aquariums
pH has an inverse relationship with the hydrogen (H⁺) concentration in the water, meaning that as the H⁺ in the solution increases, the pH decreases or becomes more acidic. As you remove H⁺, the pH increases. There are two primary factors that play a major role in determining the concentration of H⁺, and it's manipulating these factors that can control the resulting pH in reef tanks.
The first factor is carbon dioxide. CO₂ has the strongest impact on the pH of seawater. We know that only excess hydrogen ions (H⁺) can cause the pH to drop, but carbon dioxide doesn’t contain a single hydrogen ion, so how can it lower pH? When CO₂ enters the aquarium, it binds with water (H₂O) to form a weak acid called carbonic acid (H₂CO₃), which isn’t very stable in seawater. It doesn't directly drop the pH. However, carbonic acid quickly breaks apart (dissociates) into bicarbonate (HCO₃⁻) and a hydrogen ion (H⁺), and it’s that released hydrogen ion that causes the pH to drop. At first glance, it might seem like this process increases alkalinity by adding bicarbonate. However, every time you gain a bicarbonate ion, you also gain a hydrogen ion, and they effectively cancel each other out. Normal alkalinity sources supply bicarbonate or carbonate without the extra H⁺ released from carbonic acid, giving the aquarium a net increase in alkalinity.
The second factor is alkalinity. By definition, alkalinity is the amount of acid required to lower the pH of a solution to a certain level. To put it simply, it’s the water’s resistance to acidic compounds, otherwise known as the aquarium’s buffering capacity. There are several compounds that contribute to alkalinity, but the ones we care about are bicarbonate and carbonate, since those are what help corals grow and function optimally. Directly measuring the concentration of bicarbonate and carbonate is very challenging. Fortunately, the total alkalinity is predominantly composed of those ions, and it is very simple to test (Holmes-Farley, 2002). Alkalinity has a large effect on the resulting pH. When an aquarium is perfectly aerated with ambient CO₂, the resulting pH is determined entirely by the alkalinity (Holmes-Farley, 2016). Higher alkalinity levels have more resistance to pH changes and drops than lower levels (Holmes-Farley, 2004).
This graph, created by @rishma, visually represents the relationship between carbon dioxide, alkalinity, and pH.
The Interaction Between Alkalinity and CO₂
Indoor CO₂ levels in homes typically range between 500 and 1,000 ppm, which is considered safe for people. However, high indoor carbon dioxide can significantly lower the pH in seawater. In my own home, I’ve seen CO₂ spike to 900–1,000 ppm, and even higher during large family gatherings and additional cooking. For example, at 700 ppm CO₂ with alkalinity of 6.5 dKH (natural seawater), the aquarium’s pH after full CO₂
equilibrium would be about 7.9. Raising the alkalinity to 10 dKH at the same CO₂ level keeps the pH around 8.1. Outdoor air is usually around 400 ppm CO₂; at that level, 6 dKH maintains a pH near 8.1, while 11 dKH keeps it at around pH 8.4.
Do You Need to Raise Your pH?
The most commonly accepted pH range for reef tanks is between 7.8 and 8.6. Most aquariums will naturally fall within this range and maintain proper coral health as long as the alkalinity is within 7–11 dKH and calcium is at least 400 ppm (Holmes-Farley, 2004). Many hobbyists worry when they see a pH reading around 7.8, fearing it will harm their corals. I tried searching for a clear answer on the 'best' pH level, but even the research studies seem mixed.
Atkinson et al. (1995) maintained and grew 57 different stony corals long term at the Waikiki Aquarium in Hawaii. The incoming water ranged from pH 7.5 to 7.8, and the aquarium itself maintained a pH range of 7.6 to 8.3. Despite frequent exposure to the very low end of the scale, the corals’ growth rates were similar to the upper rates reported from the field.
Interestingly, just because some corals thrive at a certain pH doesn’t mean others will. Some species can resist acidified water by raising their internal pH at the calcification site (Venn et al., 2013), while others lack this ability or do so inefficiently (McCulloch et al., 2012). In fact, some corals may even grow better at lower pH levels (Anthony et al., 2008; Tanvet et al., 2023), while others may suffer slower growth and increase their risk of bleaching (Anthony et al., 2008).
Coralline algae, in particular, seem to be especially vulnerable to reduced pH. (Cornwall et al., 2022; Kuffner et al., 2008; Ragazzola et al., 2012). Overall, studies seem to show that higher pH is generally better for coral growth. For example, daytime calcification in acropora millepora increased by 178% at pH 8.4 compared to pH 8.1 under normal oxygen conditions (Wijgerde et al., 2014). Additionally, stylophora pistillata grew best at pH 8.0–8.2 compared to pH 7.6, especially when the alkalinity was higher (Marubini et al., 2008). Comeau et al. (2015) found that reef communities exposed to elevated CO₂ (~1300 ppm) for eight weeks showed a 29% decrease in calcification compared to those at natural outdoor CO₂ levels (~400 ppm).
In most cases, the motivation to raise pH comes from a single goal: faster growth. If you’re filling out a new tank and want to reach maturity sooner, have a coral farm and need to speed up production, or even want an extra edge in grow-out contests, chasing growth from every angle may be desirable.
Unfortunately, ocean and lab tank studies don’t always translate to the same results we might experience in home aquariums. An example is how most reef tanks would starve and risk dinoflagellates if they emulated the inorganic nutrient levels of the ocean. While lab tanks and home aquariums both keep corals in four-cornered, glass boxes, experimental tanks usually haven’t reached the same level of maturity and husbandry that our setups contain. Possibly most inconvenient, most studies don’t test the corals commonly kept in the hobby. As we’ve seen above, even closely related species can respond very differently to the same pH conditions. Finding studies that test the corals we keep was like searching for a needle in a haystack.
Anecdotally, many hobbyists report noticeably improved coral growth after increasing the pH, and the studies generally seem to agree that higher pH can provide better growth compared to lower levels, but determining how much more growth it will influence, as we’ve seen, is tricky. Many hobbyists, especially those with mature and overgrown corals, find increasing their pH to be extra work with no space to sustain the extra growth. In those cases, it may actually work in their favor to allow the pH to settle around the lower end. Others, however, choose to go the extra mile, aiming to maximize growth wherever possible. Some believe higher pH decreases a coral's vulnerability to STN, though the evidence for that isn’t clear. Rather than debate the “ideal” number, this article will focus on understanding and managing pH from a clear, practical, and unbiased perspective.
How Elevated pH Might Make Calcification Easier
There are two main mechanisms a higher pH might improve growth. Coral skeletons are composed of calcium carbonate (CaCO₃), which requires carbonate ions as the primary building blocks. In seawater, carbonate is already in short supply compared to bicarbonate, and as pH decreases, more carbonate ions are converted into bicarbonate, further reducing their availability for calcification (Smithsonian Institution, n.d.). When carbonate is in short supply, many corals can internally convert bicarbonate into carbonate, but this process results in an extra hydrogen ion (H⁺) that the calcifying organism must expel to complete calcification. When the surrounding water already contains a high concentration of H⁺, it becomes more difficult to pump out the excess hydrogen against the gradient, and this limitation might even outweigh carbonate unavailability (Jokiel, 2013).
Challenges with Measuring pH
Before deciding whether to increase the pH, you’ll first need to know what it is. If there’s one takeaway from this article, it’s that pH is one of the most difficult parameters to measure accurately. pH naturally fluctuates throughout the day and night, and even within the hour. Photosynthesis is the primary reason why the pH trends upwards as the day progresses, reaching its peak by the very end of the photoperiod. At night, when photosynthesis stops, the pH decreases, hitting its lowest reading prior to the lights turning back on. It’s relatively common to see a 0.3 pH swing throughout a 24-hour period, so a single pH reading at a moment in time will not result in an accurate depiction of the entire pH cycle.
The pH of calibration standards isn't stable, either. While proper standards are buffered to resist shifting from carbon dioxide, they’re not fully resistant, especially the pH 10 solutions. Additionally, not all calibration standards are made accurately to begin with, and determining whether a calibration solution is adequate is often based on trust. pH testers can only be as accurate as the standards they’re calibrated with. It’s a paradox: you need an accurate buffer to calibrate your probe, but how can you determine the buffer’s accuracy if you don’t already have a calibrated probe?
Finally, pH probes can drift over time. I personally notice my probe significantly shifts after 1–2 months, but many may begin to drift even sooner. The only way to determine if the probe has lost its accuracy is to test it against standards or recalibrate it and compare the old and new values.
Choosing a pH Probe
The pH of a reef tank is always subject to change, especially between day and night, which is why I prefer inline probes that monitor the pH 24/7. I have a probe connected to my controller system, and it logs data to a graph in real time, allowing me to visualize trends as recent as the past hour up to a full week. It’s worth knowing the highs and lows of the pH cycle prior to deciding whether pH-boosting efforts are warranted.
A good probe should support two-point calibrations using pH 7 and 10 standards. Using two reference points achieves better accuracy than if the probe can only extrapolate the voltage from a one point. For the type of probe, I prefer lab-grade, double-junction probes. Unlike single-junction probes, these prevent precipitation on the porous junction by keeping the silver chloride solution away from direct contact with seawater. They tend to last longer and hold calibration better than the single-junction types.
Additionally, I prefer probes with a ±0.01 pH accuracy. This level of resolution helps determine whether pH adjustment efforts are moving in the right direction, as changes can initially be subtle. It can feel tedious if the probe isn’t accurate enough to display progress, and sometimes having better accuracy can determine whether a method is starting to make a measurable difference versus having no effect at all. Finally, although a minor detail, plastic or epoxy-bodied probes are more durable and less likely to crack during handling and calibration compared to glass-bodied ones.
That being said, I like to keep things practical and realistic. As long as you calibrate regularly and check against reliable standards, even a basic pH probe can be useful. The safe range for reef tanks is fairly broad, so high precision mainly matters when you’re actively trying to maintain a specific target. The only thing I’d avoid using is pH test kits or strips. Holmes-Farley (2016) notes that test kits relying on color indicator dyes aren’t the best way to measure pH in reef aquariums. They can be inaccurate, difficult to interpret, may not cover the ideal range, degrade over time, and can’t be verified against a known standard. In my observations, the most extreme cases of low or high readings almost always stem from dye-based test kits, so I’d avoid them entirely.
Choosing pH Calibration Standards
pH buffers used for two-point calibrations should bracket the tested range. The optimal pH for reef tanks is between 7.8 and 8.6, which is why pH 7 and 10 buffers are recommended for calibration. pH 4 and 7 buffer solutions are available, but those are better suited for acidic environments, such as in calcium reactors. Calibration solutions come in several types of packaging: bottles, single-use foil packets, and even tablets one can dissolve in water at home. Individually sealed packets ensure each buffer is fresh, with no air exposure prior to use, and that’s what I use. Bottled pH solutions can work, as long as they are closed immediately after use and the probe is not inserted directly into the bottle. Opening and closing a bottle, or forgetting to recap it, can allow CO₂ to enter, altering its stated pH. In addition to air exposure, temperature alone can change pH, which is why most buffers include a reference chart showing the optimal temperature during calibration. Lastly, any pH solution, even when stored correctly, will eventually degrade as CO₂ slowly seeps in. While a very slow process, even individually sealed single-use packets aren't immune to this, and that’s why most quality buffers include an expiration date. Using inaccurate calibration solutions will result in an inaccurate probe. To put it in perspective, I’ve personally run some calculations showing a couple of scenarios:
If the pH 7 solution is actually 7.4 and the pH 10 solution is accurate, a true pH of 8.0 would appear as 7.7 (false low).
If the pH 7 solution is 7.5 and the pH 10 solution is 10.5, a true pH of 8.0 would read as 7.5 (false low)
If the pH 7 solution is accurate and the pH 10 solution is actually 9.5, a true 8.0 would read as 8.2 (false high).
If the pH 7 is actually 6.5 and the pH 10 solution is actually 9.5, a true pH of 8.0 would appear as 8.5 (false high).
Since pH 10 solutions are more likely to be lower, rather than above, their stated level, it appears that false highs might be more probable from inaccurate pH 10 calibration standards. Fortunately, many of the available commercial buffers seem to be accurate, at least they were when Holmes-Farley (2005) tested them. They were all within ±0.1 of the set standard. In no particular order, the companies whose buffers passed were Hanna, Milwaukee, Thermo Fisher, and Pinpoint. Of course, that test was done decades ago, but I personally feel comfortable using any of the above brands. Other commercial brands purchased online may work, as well.
How to Calibrate a pH Probe
Calibrating a pH probe is relatively simple, but there are slight variations depending on the type of probe being used. Some models allow you to input specific calibration values, for example, pH 7.54 instead of a flat pH 7.0. In those cases, you can initially calibrate their probes with normal 7 and 10 pH calibration solutions. Afterwards, they have the ability to test the pH of older calibration bottles lying around, mark their exact pH readings, and then use those specific pH solutions for future calibrations. However, most common probes require calibrating with exactly pH 7.0 and 10.0, with no option to deviate. This tutorial is written with those probes in mind.
To run the calibration, you’ll need a pH probe (and controller, if applicable), pH 7.0 and 10.0 calibration solutions around their optimally stated temperature, a cup of RO/DI or distilled water, and scissors, if you're using individually sealed packets.
- Swirl the probe in the cup of RO/DI or distilled water to remove any residual seawater that could alter the incoming calibration fluid.
- Remove the probe from the cup and flick/shake off the excess water. A trace of water is no problem. Try to avoid accidentally hitting the probe when shaking it off.
- Insert the probe into the pH 7.0 solution and gently swirl it for a few seconds to equilibrate it to the standard. Let it sit until the reading settles, then confirm the 7.0 calibration when prompted.
- Rinse the probe again in the cup of RO/DI water, flicking off the excess as before, then place it into the pH 10.0 solution.
- Swirl gently for a few seconds to equilibrate, but avoid vigorous agitation, which can introduce CO₂ and reduce its stated pH. Wait until the reading stabilizes, then confirm the value.
- Once complete, verify if the probe is correctly reading the pH 10 and pH 7 solutions used for calibrating.
The Aeration Test
Low pH issues in aquariums are almost always caused by excess CO₂, and there are two possible sources: the aquarium itself or elevated indoor carbon dioxide. An inexpensive CO₂ meter can determine if the ambient air is the culprit, but it won’t readily tell whether the aquarium itself is also contributing to the problem. Fortunately, there’s a straightforward, low-cost diagnostic you can run at home in just a couple of hours that can identify the exact source, and it's called The Aeration Test (Holmes-Farley 2004). The test singles out the source by aerating the 'aquarium' in different conditions and measuring the pH's response to them. Below is my summary of the method.
To run the test, you’ll need an 8 oz cup of tank water, a calibrated pH probe, a timer, an air pump with an air stone, and access to the outdoors. Only use a small cup because off-gassing carbon dioxide is very time-consuming. A small sample ensures the water will be fully equilibrated after the set time for accurate results.
- Measure the aquarium’s initial pH.
- Take a cup of tank water and heavily aerate it indoors for an hour. Record the pH.
- Take the cup and repeat the same process outdoors for an hour. Record the pH.
1) If heavily aerating a cup indoors increases the pH, there is either not enough aeration or some filtration/maintenance routine is adding too much CO₂ or H⁺.
2) If aerating the cup indoors doesn’t meaningfully raise the pH, but aerating it outdoors does, the culprit is excess indoor carbon dioxide entering the tank. The solution is to increase the pH-boosting methods ('Managing pH in Reef Tanks: Part Two' will discuss how).
3) If both the indoor and outdoor cup pH values remain low after the test, or if the outdoor sample rises, but not meaningfully, then the probe, test kit, calibration, or buffer standards are very likely inaccurate. The outdoor aeration part of the test determines if the probe is accurately responding to low CO₂ conditions and shows how concentrated the indoor CO₂ is.
Conclusion
Understanding pH in Reef Tanks: Part one primes us for the next section. In order to know how to effectively control pH, you'd first need to understand how pH works and why it behaves the way it does. Once you see it, it becomes crystal clear what makes a pH-boosting method effective, and you can even determine which methods fall short. In the next part, we'll go over all of the potential causes of low pH issues, how to eliminate them, and I'll highlight exactly how to raise pH and never allow it to be low again, if desired. We'll also go over some precipitation problem-solving. By the time you finish the second part, you might be struggling with pH that is too high.
References
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