Optimal Parameters for a Coral Reef Aquarium: By Randy Holmes-Farley

[note: edited on 5/29/2022 to update the nitrate and phosphate sections with higher recommended levels.]

One of the main roles of an aquarist with a coral reef aquarium is to ensure that the conditions are right for their tank inhabitants. There are many different attributes of the aquarium that need to be controlled, including lighting, water flow, temperature, and the concentrations of the many chemicals in the water. This article focuses on water chemistry issues, showing my recommendations for the most important of the various chemical parameters in a reef aquarium.

Table 1 shows a summary of some of the most important water parameters for reef aquaria. Table 2 shows some of the less critical parameters, or those too complicated for many aquarists to carefully control, but about which many aquarists have concerns or questions. The remainder of this article provides the rational and further discussion for each of the parameters in these tables.

Some aquarists have begun to focus more on the measurement of trace elements (i.e., those that are present at very low levels, such as iron or copper). With the exception of iron, which has a long history of utility in dosing, I will not go into these other trace elements at this time because the methods to measure and control them are not as simple as the other ions in this paper.

Table 1. Parameters critical to control in reef aquaria.


Table 2. Other parameters in reef aquaria that aquarists may want to control.


* I do not generally recommend measuring and controlling these parameters, but if you do, these are the guidelines.

Critical Parameters


Many corals use calcium to form their skeletons, which are composed primarily of calcium carbonate. The corals get most of the calcium for this process from the surrounding water. Consequently, calcium often becomes depleted in aquaria housing rapidly growing corals, calcareous red algae (coralline algae), Tridacnids (clams) and Halimeda (a macroalgae containing calcium carbonate). As the calcium level drops below 360 ppm, it becomes progressively more difficult for these organisms to collect enough calcium, thus stunting their growth.

Maintaining the calcium level is one of the most important aspects of coral reef aquarium husbandry. Most reef aquarists try to maintain approximately natural levels of calcium in their aquaria (~420 ppm). It does not appear that boosting the calcium concentration above natural levels enhances calcification (i.e., skeletal growth) in most corals.

For these reasons, I suggest that aquarists maintain a calcium level between about 380 and 450 ppm, although higher is generally not a problem until it gets so high that calcium carbonate precipitation becomes problematic. Aquarists with a very light demand may be able to maintain calcium with water changes, especially since some salt mixes have excessive calcium in them. But most established aquaria with growing hard corals and coralline algae will require some calcium supplementation, and in some cases, it might be needed every day.

I usually suggest using a balanced calcium and alkalinity additive system for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part or three-part additive systems for calcium and alkalinity. If calcium is depleted and needs to be raised significantly, however, such balanced methods are not a good choice since they will raise alkalinity too much. In that case, adding calcium chloride is a good method for raising calcium in a one-time correction.


Like calcium, many corals also use "alkalinity" to form their skeletons, which are composed primarily of calcium carbonate. It is generally believed that corals take up bicarbonate, convert it into carbonate, and then use that carbonate to form calcium carbonate skeletons. That conversion process is shown as:

HCO3- → CO3-- + H+

Bicarbonate → Carbonate + proton (which is released from the coral)

To ensure that corals have an adequate supply of bicarbonate for calcification, aquarists could just measure bicarbonate directly. Designing a test kit for bicarbonate, however, is somewhat more complicated than for alkalinity. Consequently, the use of alkalinity as a surrogate measure for bicarbonate is deeply entrenched in the reef aquarium hobby.

So, what is alkalinity? Alkalinity in a marine aquarium is simply a measure of the amount of acid (H+) required to reduce the pH to about 4.5, where all bicarbonate is converted into carbonic acid as follows:

HCO3- + H+ → H2CO3

The amount of acid needed is equal to the amount of bicarbonate present, so when performing an alkalinity titration with a test kit, you are “counting†the number of bicarbonate ions present. It is not, however, quite that simple since some other ions also take up acid during the titration. Both borate and carbonate also contribute to the measurement of alkalinity, but the bicarbonate dominates these other ions since they are generally lower in concentration than bicarbonate. So knowing the total alkalinity is akin to, but not exactly the same as, knowing how much bicarbonate is available to corals. In any case, total alkalinity is the standard that aquarists use for this purpose.

Unlike the calcium concentration, it is widely believed that certain organisms calcify more quickly at alkalinity levels higher than those in normal seawater. This result has also been demonstrated in the scientific literature, which has shown that adding bicarbonate to seawater increases the rate of calcification in some corals. Uptake of bicarbonate can consequently become rate limiting in many corals. This may be partly due to the fact that the external bicarbonate concentration is not large to begin with (relative to, for example, the calcium concentration, which is effectively about 5 times higher).

For these reasons, alkalinity maintenance is a critical aspect of coral reef aquarium husbandry. In the absence of supplementation, alkalinity will rapidly drop as corals use up much of what is present in seawater. Water changes are not usually sufficient to maintain alkalinity unless there is very little calcification taking place. Most reef aquarists try to maintain alkalinity at levels at or slightly above those of normal seawater, although exactly what levels different aquarists target depends a bit on the goals of their aquaria.

Interestingly, because some corals may calcify faster at higher alkalinity levels, and because the abiotic (nonbiological) precipitation of calcium carbonate on heaters and pumps also rises as alkalinity rises, the demand for alkalinity (and calcium) rises as the alkalinity rises. So an aquarist generally must dose more calcium and alkalinity EVERY DAY to maintain a higher alkalinity (say, 11 dKH) than to maintain 7 dKH. It is not just a one-time boost that is needed to make up that difference. In fact, calcification gets so slow as the alkalinity drops below 6 dKH that reef aquaria rarely get much below that point, even with no dosing: natural calcification has nearly stopped at that level.

In general, I suggest that aquarists maintain alkalinity between about 7-11 dKH (2.5 and 4 meq/L; 125-200 ppm CaCO3 equivalents). Many aquarists growing SPS corals and using Ultra Low Nutrient Systems (ULNS) have found that the corals suffer from “burnt tips†if the alkalinity is too high or changes too much. It is not at all clear why this is the case, but such aquaria are better served by alkalinity in the 7-8 dKH range.
As mentioned above, alkalinity levels above those in natural seawater increase the abiotic precipitation of calcium carbonate on warm objects such as heaters and pump impellers, or sometimes even in sand beds. This precipitation not only wastes calcium and alkalinity that aquarists are carefully adding, but it also increases equipment maintenance requirements and can “damage†a sand bed, hardening it into a chunk of limestone. When elevated alkalinity is driving this precipitation, it can also depress the calcium level. An excessively high alkalinity level can therefore create undesirable consequences.

I suggest that aquarists use a balanced calcium and alkalinity additive system of some sort for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part/three part additive systems.

For rapid alkalinity corrections, aquarists can simply use baking soda (sodium bicarbonate) or washing soda (sodium carbonate; baked baking soda) to good effect. The latter raises pH as well as alkalinity while the former has a very small pH lowering effect. Mixtures can also be used, and are what many hobby chemical supply companies sell as “buffersâ€. Most often, sodium carbonate is preferred, however, since most tanks can be helped by a pH boost.


There are a variety of different ways to measure and report salinity, including conductivity probes, refractometers, and hydrometers. They typically report values for specific gravity (which has no units) or salinity (in units of ppt or parts per thousand, roughly corresponding to the number of grams of dry salt in 1 kg of the water), although conductivity (in units of mS/cm, milliSiemens per centimeter) is sometimes used.

Somewhat surprisingly, aquarists do not always use units that naturally follow from their measurement technique (specific gravity for hydrometers, refractive index for refractometers, and conductivity for conductivity probes) but rather use the units interchangeably.

For reference, natural ocean water has an average salinity of about 35 ppt, corresponding to a specific gravity of about 1.0264 and a conductivity of 53 mS/cm. It often ranges from 34-36 ppt over reefs, but can be higher or lower locally for various reasons such as land run off of fresh water, or evaporation from a lagoon.

As far as I know, there is little real evidence that keeping a coral reef aquarium at anything other than natural levels is preferable. It appears to be common practice to keep marine fish, and in many cases reef aquaria, at somewhat lower than natural salinity levels. This practice stems, at least in part, from the belief that fish are less stressed at reduced salinity. I have no idea if that is true or not, but I’ve not seen evidence that it is true. Substantial misunderstandings have also arisen in the past among aquarists as to how specific gravity really relates to salinity and density, especially considering temperature effects. For example, the density of seawater is less than the specific gravity, and measurements with glass hydrometers may require temperature correction, but newer devices do not need the aquarist to make corrections. Consequently, older salinity or “specific gravity†recommendations may not actually be referring to the same measurements that aquarists make today, even if the recommended numbers have been handed down

My recommendation is to maintain salinity at a natural level. If the organisms in the aquarium are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense. Otherwise, I suggest targeting a target salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm).


Temperature impacts reef aquarium inhabitants in a variety of ways. First and foremost, the animals' metabolic rates rise as temperature rises. They may consequently use or produce more oxygen, carbon dioxide, nutrients, calcium, and alkalinity at higher temperatures. This higher metabolic rate can also increase both their growth rate and waste production at higher temperatures.

Another important impact of temperature is on the chemical aspects of the aquarium. The solubility of dissolved gases such as oxygen and carbon dioxide, for example, change with temperature. Oxygen, in particular, can be a concern because it is less soluble at higher temperature.

So what does this imply for aquarists?

In most instances, trying to match the natural environment in a reef aquarium is a worthy goal. Temperature may, however, be a parameter that requires accounting for the practical considerations of a small closed system that might suffer a power failure and trap the organisms in a small amount of poorly aerated water, something that rarely happens on a natural reef. Looking to the ocean as a guide for setting temperatures in reef aquaria may also present complications because corals grow well in such a wide range of temperatures. The greatest variety of corals, however, are found in water whose average temperature is about 83-86° F.
During normal functioning of a reef aquarium, the oxygen level and the metabolic rate of the aquarium inhabitants are not often important issues, and many reef tanks do well with temperatures in the low to mid 80's. During a crisis such as a power failure, however, the dissolved oxygen can be rapidly used up. Lower temperatures not only allow a higher oxygen level before an emergency, but will also slow the consumption of that oxygen by slowing the metabolism of the aquarium's inhabitants. The production of ammonia as organisms begin to die may also be slower at lower temperatures. For reasons such as this, one may choose to strike a practical balance between temperatures that are too high (even if corals normally thrive in the ocean at those temperatures), and those that are too low.

These natural guidelines leave a fairly wide range of acceptable temperatures. I keep my aquarium at about 80-81° F year-round. I am actually more inclined to keep the aquarium cooler in the summer, when a power failure would most likely warm the aquarium, and higher in winter, when a power failure would most likely cool it. All things considered, I recommend temperatures in the range of 76-83° F unless there is a very clear reason to keep it outside that range.

One additional comment on temperatures: having a small temperature swing is not necessarily undesirable. While temperature stability may sound like a desirable attribute, and in some cases it may be, studies have shown that organisms that are acclimated to daily temperature swings become more able to deal with unexpected temperature excursions. So while a tank creature that normally experiences only 80° F may be very healthy, the same organism adapted to a range from 78° F to 82° F may be better able to deal with an aquarium that accidentally rises to 86° F


pH is a measure of the concentration of protons (H+ ions) and hydroxide (OH-) ions in the water. Aquarists spend a considerable amount of time and effort worrying about, and attempting to solve, apparent problems with the pH of their aquaria. Some of this effort is justified, as true pH problems can lead to poor animal health. In many cases, however, the only problem is with the pH measurement or its interpretation. Moreover, the maintenance of appropriate alkalinity in seawater goes a long way to ensuring that the pH is acceptable, with just a couple of exceptions that will be discussed below.

Several factors make monitoring a marine aquarium's pH level useful. One is that aquatic organisms thrive only in a particular pH range, which varies from organism to organism. It is therefore difficult to justify a claim that a particular pH range is "optimal" in an aquarium housing many species. Even natural seawater's pH (8.0 to 8.3) may be suboptimal for some of its creatures, but it was recognized more than eighty years ago that pH levels different from natural seawater (down to 7.3, for example) are stressful to fish. Additional information now exists about optimal pH ranges for many organisms, but the data are inadequate to allow aquarists to optimize pH for most organisms which interest them.

Additionally, pH's effect on organisms can be direct, or indirect. The toxicity of metals such as copper and nickel to some aquarium organisms, such as mysids and amphipods, is known to vary with pH. Consequently the acceptable pH range of one aquarium may differ from another aquarium, even if they contain the same organisms, but have different concentrations of metals.

Changes in pH nevertheless do substantially impact some fundamental processes taking place in many marine organisms. One of these fundamental processes is calcification, or deposition of calcium carbonate skeletons, which is known to depend on pH, usually dropping as pH falls. At a low enough pH (somewhere below pH 7.7) coral skeletons can begin to slowly dissolve. Using this type of information, along with the integrated experience of many hobbyists, we can develop some guidelines about what is an acceptable pH range for reef aquaria, and what values push the limits.

The acceptable pH range for reef aquaria is an opinion rather than a clear fact, and will certainly vary with the opinion's provider. This range may also be quite different from the "optimal" range. Justifying what is optimal, however, is much more problematic than is justifying that which is simply acceptable, so we will focus on the latter. As a goal, I'd suggest that the pH of natural seawater, about 8.2, is appropriate, but coral reef aquaria can clearly succeed in a wider range of pH values. In my opinion, the pH range from 7.8 to 8.5 is an acceptable range for reef aquaria.

In truth, many aquarists never measure pH, and many that do so do not do anything with the results they obtain. This lack of action is usually okay, as most aquaria do not naturally fall outside of the acceptable ranges. Times when it is most important to at least check pH once in a while are:

1. When using very high pH additives, such as limewater (kalkwasser). In this case, one should ensure that the pH does not get above about 8.55. At higher values, the precipitation of calcium carbonate on pumps and such can become excessive. Every 0.3 pH unit rise in pH is equivalent to about a doubling of the calcium or alkalinity value in terms of the likelihood of precipitation of calcium carbonate (because bicarbonate turns into carbonate as the pH rises, driving precipitation). Aquaria may often get to a pH that is high enough to double the precipitation rate due to elevated pH, but one does not often see aquaria with calcium or alkalinity that is double the normal value, making high pH a big driver of precipitation.

2. When the air around the aquarium has elevated carbon dioxide levels, such as in a newer, tighter home. Low pH due to elevated carbon dioxide in the air is VERY common. While it may be useful to ensure the pH stays above 8.0, there are many fine aquaria with the bottom end of the pH range at pH 7.8. Below that value, I'd want to take more aggressive action, such as more fresh air in the home, top off with limewater (kalkwasser), a fresh air line from outside to a skimmer inlet, or a CO2 scrubber on a skimmer inlet.


Magnesium's primary importance is its interaction with the calcium and alkalinity balance in reef aquaria. Seawater and reef aquarium water are always supersaturated with calcium carbonate. That is, the solution's calcium and carbonate levels exceed the amount that the water can hold at equilibrium. How can that be? Magnesium is a big part of the answer. Whenever calcium carbonate begins to precipitate, magnesium binds to the growing surface of the calcium carbonate crystals. The magnesium effectively clogs the growing crystal surface so that they no longer look like calcium carbonate, making it unable to attract more calcium and carbonate, so the precipitation stops. Without the magnesium, the abiotic (nonbiological) precipitation of calcium carbonate would likely increase enough to prohibit the maintenance of calcium and alkalinity at natural levels.

For this reason, I suggest targeting the natural seawater concentration of magnesium: ~1285 ppm. For practical purposes, 1250-1350 ppm is fine, and levels slightly outside that range (1250-1400 ppm) are also likely acceptable. Higher levels may be fine, but there is no reason to keep it higher, with the possible exception of trying to kill bryopsis with certain magnesium supplements (which may work due to an impurity rather than the magnesium itself). I would not suggest raising magnesium by more than 100 ppm per day under normal conditions, in case the magnesium supplement contains any toxic impurities. If you need to raise it by several hundred ppm, spreading the addition over several days will allow you to more accurately reach the target concentration, and might possibly allow the aquarium to handle any impurities that the supplement contains (such as ammonia or trace metals).

An aquarium's corals and coralline algae can deplete magnesium by incorporating it into their growing calcium carbonate skeletons. Many methods of supplementing calcium and alkalinity may not deliver enough magnesium to maintain it at a normal level. Settled limewater (kalkwasser), for example, is quite deficient in magnesium relative to a coral skeleton. Consequently, magnesium should be measured occasionally, particularly if the aquarium's calcium and alkalinity levels seem difficult to maintain. Aquaria with excessive abiotic precipitation of calcium carbonate on objects such as heaters and pumps might suffer from low magnesium levels (along with high pH, calcium, and alkalinity). In general, magnesium is usually depleted at roughly 10% of the rate of calcium depletion, or less, depending on the creatures in the aquarium. Any depletion rate that is much higher than that is either due to testing errors, or water changes with a mix that has a different magnesium level than the aquarium.

Many people never need any magnesium supplements. Some salt mixes start so high that it will never drop below natural levels, and some calcium and alkalinity supplement methods, such as a good quality two part system, add enough magnesium that it should not decline.


The "simplest" form of phosphorus in reef aquaria and in natural seawater is inorganic orthophosphate (H3PO4, H2PO4-, HPO4--, and PO4--- are all forms of orthophosphate). Inorganic orthophosphate is the only form of phosphorus that most test kits measure, including the misnamed Hanna "phosphorus" checker. Almost none of these kits measure organic phosphate, as is present in proteins, DNA, and phospholipids.

The inorganic phosphate concentration in seawater varies greatly from place to place, and also with depth and with the time of day. Surface waters are greatly depleted in phosphate relative to deeper waters, due to biological activities in the surface waters that sequester phosphate in organisms. Typical ocean surface phosphate concentrations are very low by typical reef keeping standards, sometimes as low as 0.005 ppm.

Absent of specific efforts to minimize the phosphate level, it will typically accumulate and rise in reef aquaria. It is introduced mostly with foods, but can also enter with top-off water and in some methods of calcium and alkalinity supplementation. Rarely does one need to look beyond foods as the primary source, however.

If allowed to rise above natural levels, phosphate can cause two undesirable results. One is inhibition of calcification. That is, it can reduce the rate at which corals and coralline algae can build calcium carbonate skeletons, potentially stunting their growth.

Phosphate can also be a limiting nutrient for algae growth. If phosphate is allowed to accumulate, algae growth may become problematic. At concentrations below about 0.03 ppm, the growth rate of many species of phytoplankton, for example, depends on the phosphate concentration (assuming that something else is not limiting growth, such as nitrogen or iron). Above this level, the growth rate of many of the ocean's organisms is independent of phosphate concentration (although this relationship is more complicated in a reef aquarium containing iron and/or nitrogen sources such as nitrate above natural levels). So deterring algae growth by controlling phosphate requires keeping phosphate levels quite low.

For these reasons, I recommend that phosphate should be kept at 0.02 to 0.1 ppm. It is possible to drive phosphate too low, which can result in pale corals and may be a risk factor for pest dinoflagellates. In such a case, allowing phosphate to rise a bit, or providing the corals other foods, may be very useful. On the other hand, while they are few and far between, there are a small number of very nice aquaria with VERY high phosphate levels (above 1.0 ppm). Exactly how these aquaria avoid the problems that other aquaria suffer at high phosphate is unknown.

The best ways to maintain appropriate levels of phosphate in normal aquaria are to incorporate some combination of phosphate export mechanisms, such as growing and harvesting turf algae, macroalgae or other rapidly growing organisms, using foods without excessive phosphate, skimming, using limewater, using phosphate binding media such as GFO (granular ferric oxide; always brown or black) and using organic carbon dosing (e.g., vodka, vinegar, biopellets, etc.) to drive bacterial growth. Dosing of phosphate may be useful in some aquaria, especially those with a lot of fresh bare calcium carbonate surfaces that can bind phosphate.


Ammonia (NH3) is excreted by most reef tank animals and some other aquarium inhabitants. Unfortunately, it is very toxic to all animals, although it is not toxic to certain other organisms, such as some species of macroalgae that readily consume it. Fish are not, however, the only animals that ammonia harms, and even some algae are harmed by less than 0.1 ppm ammonia.

In an established reef aquarium, the ammonia produced is usually taken up rapidly. Macroalgae use it to make proteins, DNA, and other biochemicals that contain nitrogen. Bacteria also take it up and convert it to nitrite, nitrate, and nitrogen gas (the famous "nitrogen cycle"). All of these compounds are much less toxic than ammonia (at least to fish), so the ammonia waste is rapidly "detoxified" under normal conditions.

Under some conditions, however, ammonia may be a concern. During the initial setup of a reef aquarium, or when new live rock or sand is added, an abundance of ammonia may be produced that the available mechanisms cannot detoxify quickly enough. In these circumstances, fish are at great risk. Ammonia levels as low as 0.2 ppm can be dangerous to fish. In such instances, the fish and invertebrates should be removed to cleaner water, or the aquarium treated with an ammonia-binding product such as Amquel or Prime.

Many aquarists are confused by the difference between ammonia and a form of it that is believed to be less toxic: ammonium. These two forms interconvert very rapidly (many times per second), so for many purposes they are not distinct chemicals. They are related by the acid base reaction shown below:

NH3 + H+ ←→ NH4+

Ammonia + hydrogen ion (acid) ←→ ammonium ion

The reason that ammonium is thought to be less toxic than ammonia is that, being a charged molecule, it crosses fish gills and enters their bloodstream more slowly than does ammonia, which readily passes across the gill membranes and rapidly enters the blood.

In aquaria with higher pH levels, which contain less H+, more of the total ammonia will be in the NH3 form. Consequently, the toxicity of a solution with a fixed total ammonia concentration rises as pH rises. This is important in such areas as fish transport, where ammonia can build to toxic levels.

Some reefkeepers dose ammonia, and that's a great plan as long as the concentration is ensured to not rise above target levels.

Recommendation Details: Other Parameters


Potassium is listed with the less critical parameters, not because it isn't important, but due to the fact that it does not get rapidly depleted in most aquaria. The majority of reef aquarists do not test for or dose potassium, and likely have adequate amounts from water changes alone. Potassium is important for cellular function, and generally is higher in concentration inside of cells than outside. In people, for example, nearly all of it is inside of cells, with very low concentrations present in the blood.

In marine systems, most cells of organisms have higher concentration of potassium in them than the surrounding seawater. That would make it seem that potassium would be depleted rapidly as organisms grow and add tissue mass, whether they are bacteria, microalgae, macroalgae, fish, or corals. However, there is quite a lot of potassium in seawater and salt mixes, and aquaria are typically being feed foods that also consist largely of cells that once contained potassium. Assuming these cells are not broken open and rinsed free of potassium, a large amount comes in with foods. So the net concentration of potassium in the tank will be a balance between the food and other inputs, and the uptake from tissue mass (whether it is exported or left in the tank).

A number of aquarists have found their aquaria are depleted in potassium and dose it to maintain natural levels. I've not found it to be depleted in my aquarium and I do not dose any. Some people associate depletion with organic carbon dosing to drive bacterial growth, but I've not seen that in my system (perhaps due to the foods that I choose to feed). Of those with depleted potassium, the primary symptom seems to be certain issues with SPS corals such as Montipora. Sometimes it is reported as poor growth and/or greyish coloration. I do not know if that really does relate to low potassium, but if you have such an issue, measuring potassium with a kit and dosing if necessary may be useful. I'd recommend maintaining about 380-420 ppm, but if it already is higher than that level, I would not do anything to try to lower it.


Silica raises two issues. If diatoms are an ongoing problem in an established reef aquarium, they may indicate a substantial source of soluble silica, especially tap water, since diatoms need silica to survive. In that case, purifying the tap water will likely solve the problem. In such a situation, testing may not reveal elevated silica levels because the diatoms may use it as quickly as it enters the aquarium.

If diatoms are not a problem, then I suggest that many aquarists should consider dosing silicate (a more soluble form of silica). Why would I recommend dosing silica? Largely because creatures in our aquaria use it, the concentrations in many aquaria are below natural levels, and consequently the sponges, mollusks, and diatoms living in these aquaria may not be getting enough silica to thrive.

I suggest dosing sodium silicate solution, as it is a readily soluble form of silica. I dose a bulk grade of sodium silicate solution (water glass), which is very inexpensive. You may find "water glass†in stores or online because consumers use it for such activities as preserving eggs.

Based on my dosing experience, aquarists are probably safe dosing to 1 ppm SiO2 once every 1-2 weeks. This is based on the fact that my aquarium uses that much in less than four days without any sort of "bad" reaction. Of course, there's nothing wrong with starting at a tenth of that dosage and gradually ramping it up. If you do get too many diatoms, just back off on the dosing. I presume that all of the SiO2 I have added to my aquarium has been used by various organisms (sponges, diatoms, etc.), but perhaps I have more sponges than other aquarists. Consequently, diatoms may be more of a concern in some aquaria than in mine. GFO (granular ferric oxide) used to reduce phosphate also tends to bind silicate and reduce its concentration.

If you choose to dose silicate, I would also advise occasionally measuring the soluble silica concentration in the water, in case the demand in your aquarium is substantially less than mine. If the concentration started to rise above 3 ppm SiO2, even in the absence of diatoms, I would probably reduce the dosing rate because that is close to the maximum concentration that surface seawater ever contains.


I do not presently dose iodine to my aquarium, and do not recommend that others necessarily do so without verifying for themselves that it is useful in their tank. Iodine dosing is more complicated than dosing other ions due to its substantial number of different naturally existing forms, the number of different forms that aquarists actually dose, the fact that all of these forms can interconvert in reef aquaria, and the fact that the available test kits often detect only a subset of the total forms present. This complexity, coupled with the fact that no commonly kept reef aquarium species are known 9in the scientific literature) to require significant iodine, suggests that dosing is possibly unnecessary and problematic.

I dosed iodide for years, and then stopped and never saw any difference in any creatures I kept (including macroalgae, shrimp, etc., but I obviously have never kept every possible creature that others may keep). Many others have reproduced that finding. Still others, however, are convinced that iodine is useful in their aquaria.

Iodine in the ocean exists in a wide variety of forms, both organic and inorganic, and the iodine cycles between these various compounds are very complex and are still an area of active research. The nature of inorganic iodine in the oceans has been generally known for decades. The two predominate forms are iodate (IO3-) and iodide (I-). Together these two iodine species usually add up to about 0.06 ppm total iodine, but the reported values vary by a factor of about two. In surface seawater, iodate usually dominates, with typical values in the range of 0.04 to 0.06 ppm iodine. Likewise, iodide is usually present at lower concentrations, typically 0.01 to 0.02 ppm iodine.

Organic forms of iodine are any in which the iodine atom is covalently attached to a carbon atom, such as methyl iodide, CH3I. The concentrations of these organic forms (of which there are many different molecules) are only now becoming recognized by oceanographers. In some coastal areas, organic forms can comprise up to 40% of the total iodine, so many previous reports of negligible levels of organoiodine compounds may be incorrect.

The primary organisms in reef aquaria that "use" iodine, at least as far as are known in the scientific literature, are algae (both micro and macro). My experiments with Caulerpa racemosa and Chaetomorpha sp. suggest that iodide additions do not significantly increase the growth rate of these macroalgae, which are commonly used in reef aquaria. Other macroalgae species may respond differently, but none are known in the scientific literature to “need†iodine.

Finally, for those interested in dosing iodine, I suggest that iodide is the most appropriate form for dosing. Iodide is more readily used by some organisms than is iodate, and it is detected by test kits. While many people use it and are happy with the results, I am not a fan of Lugols iodine (a mixture of I2 and I-) because it is reactive and unnatural. With that as a backdrop, my recommendation is to experiment with iodine if you want, but be ready for there to be no benefit and to stop if that seems the case. For reasons relating to the complexity of iodine forms and testing, I usually advise aquarists to not try to maintain a specific iodine concentration using supplementation and test kits, but to dose something like a NSW equivalent once a week or so.

I would also avoid commercial timed release iodine products. I do not know what any of these products actually are, but most likely they are an organoiodine form of some sort. There is little data available on the effects of such compounds in aquaria, and I see no reason to experiment with them.


Nitrate is an ion that has long dogged aquarists, but recent innovations have made it much less of a chronic problem. The nitrogen that forms it comes in with foods, and can, in many aquaria, raise nitrate enough to make it difficult to maintain natural levels. In the past, many aquarists performed water changes with nitrate reduction as one of their primary goals. Fortunately, we now have a large array of ways to keep nitrate in check, and modern aquaria suffer far less from elevated nitrate than did those in the past.

Nitrate is often associated with algae, and indeed the growth of algae is often spurred by excess nutrients, including nitrate. Other potential aquarium pests, such as dinoflagellates, are also spurred by excess nitrate and other nutrients. Nitrate itself is not acutely toxic at the levels usually found in reef aquaria, at least as is so far known in the scientific literature. Nevertheless, elevated nitrate levels can excessively spur the growth of zooxanthellae, which in turn can actually decrease the growth rate of their host coral, and turn them brown.

For these reasons, most reef aquarists strive to keep nitrate levels down. A good target, in my opinion, is 2-10 ppm nitrate ion. Reef aquaria can function acceptably at much higher nitrate levels (say, 20-50 ppm), but run greater risks of the problems described above. too low of levels is another potential risk factor for problem dinoflagellates.

There are many ways to reduce nitrate, including reducing the aquarium's nitrogen inputs, increasing nitrogen export by skimming, increasing nitrogen export by growing and harvesting macroalgae or turf algae (or any other organism of your choice), using a deep sand bed, live rock, removing existing filters designed to facilitate the nitrogen cycle, using a carbon denitrator, using a sulfur denitrator, using organic carbon dosing (vinegar, vodka, biopellets, etc.), using nitrate absorbing solids, and using polymers and activated carbon that bind organics before they break down. I use many of these: vinegar dosing, skimming, growing macroalgae, lots of live rock in refugia, and activated carbon.

Dosing nitrate is also a good plan if nitrate is too low. Food grade sodium nitrate or calcium nitrate is a good method and inexpensive. Note that dosing nitrate boosts alkalinity when the nitrate is consumed, adding 2.3 dKH for each 50 ppm of nitrate consumed.


Aquarists' concerns about nitrite are usually imported from the freshwater hobby. Nitrite is far less toxic in seawater than in freshwater. Fish are typically able to survive in seawater with more than 100 ppm nitrite! Unless future experiments show substantial nitrite toxicity to reef aquarium inhabitants, I do not consider nitrite to be an important parameter for reef aquarists to monitor. Tracking nitrite in a new reef aquarium can nevertheless be instructive by showing the biochemical processes that are taking place. In most cases, I do not recommend that aquarists bother to measure nitrite in established aquaria.


Strontium is another ion that was thought very important in the past, but when many people stopped dosing it, nothing appeared to happen. There certainly are still adherents to it that believe it useful in their aquaria, but like iodine, if you dose it, do so as an experiment and see if it is useful rather than assuming it is and spending a lot of time and money monitoring and controlling it.

If you choose to dose it, my recommendation is to maintain strontium levels in the range of 5-15 ppm. That level roughly spans the level in natural seawater of 8 ppm. I do not recommend that aquarists supplement strontium unless they have measured strontium and found it to be depleted. I have not evaluated any strontium test kits recently, but some of those I've used in the past were not very satisfactory. Hopefully, that has changed, but if not, it makes controlling strontium even more challenging.

In some tests that I did in the past (not using a kit but an ICP-AES instrument), I found that in my reef aquarium, without any strontium additions, strontium was already elevated above natural levels (to 15 ppm due to elevated strontium in the Instant Ocean salt mix that I was using). I would not like to see it get any higher. Consequently, adding a supplement without knowing the aquarium's current strontium level is not advisable. Scientific evidence indicates that some organisms need strontium, albeit not the organisms that most reef keepers maintain. Certain gastropods, cephalopods, and radiolaria, for example, require strontium. It is, however, clearly toxic at elevated concentrations. In one reported case, 38 ppm was enough strontium to kill a particular species of crab (Carcinus maenas). No evidence indicates that 5-15 ppm strontium is harmful to any marine organism, although it is not known what strontium levels are optimal. Finally, anecdotal evidence from a number of advanced aquarists suggests that strontium that is substantially below natural levels is detrimental to the growth of corals that many aquarists maintain, but this effect has not been proven.

Strontium can become depleted in reef aquaria because it looks chemically like calcium, and gets "accidentally" incorporated into calcium carbonate as it precipitates, either on pumps and heaters, or in coral skeletons. For many aquarists, water changes with a salt mix containing a suitable level of strontium may be the best way to keep strontium at appropriate levels.


I do not recommend that aquarists try to "control" ORP.

The oxidation reduction potential (ORP) of a marine aquarium is a measure of its water's relative oxidizing power. ORP has often been recommended to aquarists as an important water parameter, and some companies sell products (equipment and chemicals) designed to control ORP. Many who recommended ORP control have convinced aquarists that it is a measure of the aquarium water's relative "purity," despite this never having been demonstrated

ORP, at its heart, is very, very complicated. It is perhaps the single most complicated chemical feature of marine aquaria that aquarists will typically encounter. ORP involves many chemical details that are simply unknown, either for seawater or for aquaria. It involves processes that are not at equilibrium, and so are difficult to understand and predict. Even more daunting is the fact that the chemicals that control ORP in one aquarium might not even be the same chemicals that control ORP in another aquarium, or in natural seawater. In many seawater situations, the measured ORP value may be actually measuring the relative concentration of the different redox forms of various metals, such as iron and manganese.

ORP is, however, an interesting measure of the properties of water in a marine aquarium. It has uses for monitoring certain events in aquaria that impact ORP but may be otherwise hard to detect. These events could include immediate deaths of organisms, as well as long term increases in the levels of organic materials. Aquarists who monitor ORP, and who do other things that seem appropriate for maintaining an aquarium in response to the ORP value (such as increasing aeration, skimming, use of carbon, etc.) may find monitoring ORP to be a useful way to see progress.

ORP measurements are very susceptible to errors, however. Aquarists are strongly cautioned to not overemphasize absolute ORP readings, especially if they have not recently calibrated their ORP probe. Rather, ORP measurements are most useful when looking at changes in measured ORP over time.

Some aquarists use oxidizers (such a permanganate) to raise ORP, although this practice is far less common now than in the early days of reef keeping. These additions may benefit some aquaria, and may be beneficial in ways that aren't demonstrated by changes in ORP alone (for example, reducing the yellowness of the water). I've never added such materials directly to my aquarium, although I have used ozone in the past. In the absence of convincing data otherwise, such additions seem to me to be potentially riskier than is justified by their demonstrated and hypothesized benefits (except for properly used ozone).

ORP is important if you choose to use ozone as an indicator that you are not using too much and thereby putting your creatures at risk. In my opinion, however, the absolute ORP value is not even a good indicator that you are using an appropriate amount of ozone. For that purpose, the change in ORP from before to after the ozone use may be more appropriate.


Boron's importance in marine aquaria is a subject not often discussed by hobbyists, despite the fact that some people dose it daily with their commercial alkalinity supplements. Most commentary on boron, in fact, derives from manufacturers who sell it in one fashion or another as a "buffering" agent. These discussions, unfortunately, nearly always lack any quantitative discussion of boron or its effects, both positive and negative. In general, boron is not an important element to control in aquaria, in my opinion.

Boron, which is present as boric acid and borate in seawater, contributes only a minor fraction of normal seawater's pH buffering capacity, with nearly all of the buffering instead coming from the bicarbonate/carbonate buffer pair. Borate appears to be a necessary or desirable nutrient for certain organisms, but is also toxic to others at levels not far above natural levels.

For these reasons, my recommendation is to maintain approximately natural levels of boron, about 4.4 ppm boron. Any value below 10 ppm is likely acceptable for most aquaria. Values above 10 ppm should be avoided. Most reef aquaria likely get adequate boron from their salt mixes and water changes, and in general, I don't recommend that most aquarists bother to try to specifically control the level. It does not seem to be rapidly depleted in most aquaria.


The iron concentration is low enough that is limiting to the growth of phytoplankton in parts of the ocean, and may be limiting to macroalgae and turf algae in many reef aquaria. Because of its short supply and critical importance, it is also subject to aggressive sequestration by bacteria, algae, and other marine organisms. Consequently, aquarists might consider dosing iron if they grow macroalgae or have an algal turf scrubber, and possible even if they do not.

Iron is not easy to measure at levels normally encountered in the ocean or in marine aquaria. Kits cannot read low enough to detect the low levels involved, unless it is dosed in unusually large amounts. It is also not easy to determine which of its many forms are bioavailable in seawater, and which are not. Consequently, aquarists should not target a specific concentration, but rather should decide if they want to dose any at all, and then use a reasonable dosage going forward to observe the effects. The reason to dose iron is that macroalgae and turf algae may benefit from it. If you are not growing these algae, then you may not need to monitor or dose iron at all.

Deciding how much iron to add is fairly easy because, in my experience, it doesn't seem to matter too much. Presumably, once you add enough to eliminate it as a limiting nutrient, extra iron does not cause apparent harm in most cases (although there are some people who think it can encourage the growth of cyanobacteria). For years I dosed an iron citrate solution, but more recently I've switched to a solution made by dissolving one Fergon tablet (ferrous gluconate; an iron supplement for people available in drug stores) in about 20 mL of RO/DI water. The tablet disintegrates after soaking overnight. Then I shake up the mixture, let any solids settle out, and dose about 1-2 mL of this clear greenish fluid once or twice a week to my system with a total water volume of about 300 gallons.

If you buy a commercial iron supplement, I'd advise using only iron supplements that contain iron chelated to an organic molecule. The iron sold for freshwater applications is sometimes not chelated because free iron is more soluble in the lower pH of freshwater aquaria. I'd avoid those products in marine applications. It will likely still work, as many of the studies in the scientific literature use free iron in seawater, but probably not as well because it may precipitate before it has fully fortified the system with iron.

In many cases of iron products intended for the marine hobby, the product may not state what the iron is chelated with, in order to protect proprietary formulations. I don't actually know if it matters much. Very strong chelation by certain molecules will actually inhibit bioavailability by prohibiting release of the iron unless the chelating molecule is completely taken apart, but I expect that manufacturers have avoided those molecules. EDTA, citrate, and some others actually degrade photochemically, continually releasing small amounts of free iron. It is believed to be the free iron that many of the organisms actually take up.

It should be noted that iron may be a limiting factor for many organisms other than macroalgae. These might include microalgae, bacteria (even pathogenic bacteria), cyanobacteria, and diatoms. If unexpected problems should arise, backing off or stopping the iron additions may be warranted.


Chemical issues in reef aquaria are often daunting to aquarists. There are many chemical parameters that aquarists monitor, some of which are critical for success, and some of which are much less important. Of those critical for success, only calcium and alkalinity require regular supplementation in most reef aquaria, although the others in Table 1 may require monitoring. Successfully keeping the parameters in Table 1 at appropriate levels should go a long way toward allowing aquarists to more fully enjoy their aquaria while at the same time ensuring that the inhabitants are well cared for.

Happy Reefing.

For further discussion, you can ask me questions in the Reef Chemistry Forum at Reef2Reef: Reef Chemistry by Randy Holmes-Farley

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