The “How To” Guide to Reef Aquarium Chemistry for Beginners,
Part 2: What Chemicals Must be Supplemented

This article is the second in a series that deals with coral reef aquarium chemistry issues on a basic and practical level. Its primary purpose is to get new aquarists to focus on those aspects of reef aquarium chemistry that are truly important, instead of on those that are not. New aquarists are bombarded with a huge assortment of issues and opinions relating to aquarium husbandry practices, and none seems to cause more anxiety than chemistry issues. Some of these issues are actually very complicated, and the answers to many questions are simply not known. Fortunately for hobbyists, knowing the answers to these questions is rarely important to keeping a wonderful reef aquarium. Those issues that are important to understand are much more straightforward and can be solved without excessive anxiety.

The first article in this series focused on issues relating to the saltwater used in a coral reef aquarium, including selecting salt mixes, measuring salinity and purifying tap water.

This second article deals with the issue of supplements. Specifically, what should be supplemented in most reef aquaria, what might usefully be supplemented in some reef aquaria, and what apparently need not be supplemented? As with the first article, this one will not focus on details of the scientific rationale for certain supplements. Such scientific ideas do form the basis of every recommendation included here, and separate articles detailing them are linked from appropriate sections of this article for those who want such detail. Nevertheless, such a level of scientific detail is often unnecessary for most aquarists.

The full set of articles in this series is expected to include:

The “How To” Guide to Reef Aquarium Chemistry for Beginners

  • Part 1: The Salt Water Itself
  • Part 2: What Chemicals Must be Supplemented
  • Part 3: pH
  • Part 4: What Chemicals Must be Monitored to Prevent Buildup

The sections of this article are:

Introduction to Supplements

Many aquarists who have learned reef aquarium husbandry from information supplied by store employees and supplement manufacturers are often surprised that many of the products that they are sold are either unnecessary or ineffective. No part of the hobby distresses me more than the claims of some of the products sold to reef aquarists. Obviously, many additives are genuinely important to aquarists (many calcium and alkalinity additives, for example). Then there is a grey area where differences of opinion abound as to the merit of certain additives, with those differences arising in part from the fact that it isn’t always agreed upon whether certain chemicals in the water are important or not and, if they are, at what levels. Finally, there is a dark side to the hobby, with manufacturers pushing products that have no useful purpose. Aquarists should be skeptical of the claims of products, especially any that seem too good to be true. They probably are.

As a summary of what I believe to be important and what I have typically used in my reef aquarium for the past 11 years, here’s a list of supplements and their necessity. The need for some supplements is potentially tied to the performance of water changes, and the sections below assume an ordinary water change schedule of 15-30% per month or 0.5-1% daily. It also assumes that the fish in the aquarium are being fed appropriate marine foods. More details on each topic are given in subsequent sections.

Necessary Supplements in Nearly All Reef Aquaria

  • Calcium
  • Alkalinity

Useful Supplements in Many Reef Aquaria

  • Magnesium (if not maintained by calcium addition methods and water changes)
  • Iron (for aquaria growing macroalgae)
  • Silicate (for aquaria with sponges, snails, etc.)

Possibly Useful Supplements in Reef Aquaria

  • Certain amino acids (e.g., aspartic acid)
  • Fatty acid supplements

Generally Unnecessary or Undesirable Supplements in Most Reef Aquaria

  • Iodine
  • Strontium
  • Borate
  • Trace element mixtures

In this article I have not addressed vitamin supplements or similar products intended mostly to be mixed with fish foods. In general, the utility of adding these to the water itself has not been demonstrated. I’ve also not addressed those supplements which do not even make claims about what they are, except that they improve coloration, health or your fishes' (and perhaps even your own!) sex life.

Supplementing Calcium and Alkalinity

Nearly all coral reef aquaria need calcium and alkalinity supplementation because they are what hard corals, coralline algae and even some soft corals use to deposit calcium carbonate structures such as skeletons and spicules. The concept of alkalinity confuses many aquarists, but it is most readily thought of as a measure of bicarbonate ion (HCO3-) in the water, and corals take up bicarbonate to deposit calcium carbonate. Because rapidly growing corals and coralline algae can quickly deplete calcium and alkalinity (especially alkalinity because seawater contains much less alkalinity than calcium), these ions must be supplemented to the aquarium.

Calcium Target Levels

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 water surrounding them. Consequently, calcium often becomes depleted in coral reef aquaria. As the calcium level drops below 360 ppm, it becomes progressively more difficult for the corals 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. I suggest that aquarists maintain a calcium level between about 380 and 450 ppm.

Alkalinity Target Levels

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.

To ensure that corals have an adequate supply of bicarbonate for calcification, aquarists could very well just measure bicarbonate directly. Perhaps some day that will be done. For now, alkalinity is a surrogate measure that is perfectly adequate because bicarbonate greatly dominates all other ions that are added together to measure total alkalinity.

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 says that the uptake of bicarbonate can apparently become rate-limiting in the growth of some corals.

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. Most reef aquarists try to maintain alkalinity 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. Those wanting the most rapid skeletal growth, for example, often push alkalinity to higher levels. I suggest that aquarists maintain alkalinity between about 2.5 and 4 meq/L (7-11 dKH, 125-200 ppm CaCO3 equivalents), although higher levels may be acceptable as long as they do not depress the calcium level.

Alkalinity levels above those in natural seawater increase the abiotic (nonbiological) precipitation of calcium carbonate on objects such as heaters and pump impellers. This precipitation not only wastes calcium and alkalinity that aquarists are carefully adding, but it also increases equipment maintenance requirements. When elevated alkalinity is driving this precipitation, it can also depress the calcium level. A raised alkalinity level can, therefore, create undesirable consequences.

Supplementing Calcium and Alkalinity: What Method?

The best way to supplement calcium and alkalinity is with some sort of additive system that provides them in the exact ratio at which they are depleted by corals. Many beginning aquarists don’t believe this opinion, or at least have started down the road of independent calcium and alkalinity additives because that is what the local store sold them, and are hesitant to believe the importance of changing. After all, they argue, I can closely monitor calcium and alkalinity with test kits, so why does it matter if I add them in a fixed ratio? Wouldn’t it be better to add exactly what I need when I need it?

That sounds good, but in practice many real world factors get in the way. For example:

1. Test kits do not always provide accurate results (through kit or human error).
2. Aquarists quickly tire of measuring water parameters, and slack off.
3. Accidental over- or under dosing is much more common than might be thought.

After answering many tens of thousands of reef aquarium chemistry questions for hobbyists over the past decade, it is clear that calcium and alkalinity problems dominate their chemistry concerns (with pH a close second). Many of these calcium and alkalinity problems start off with sentences such as, “My calcium is okay, but my alkalinity is off the charts.” While using balanced calcium and alkalinity additives does not prevent problems from arising, it does greatly cut down on the list of typically encountered problems, and those that remain are usually easier to solve.

There are many good ways to provide a balanced ratio of calcium and alkalinity. Before discussing the methods, let me explain a bit more about what a balanced ratio actually is. Pure calcium carbonate contains one calcium ion for each carbonate ion. That ratio corresponds to 1 meq/L of alkalinity (= 2.8 dKH or 50 ppm calcium carbonate equivalents) for each 20 ppm of calcium. In a real coral skeleton, some magnesium ions get into spaces ordinarily occupied by calcium, so the real ratio in corals and coralline algae is usually closer to 18-20 ppm calcium for each 1 meq/L of alkalinity, and that ratio varies a bit by organism.

The balanced calcium and alkalinity methods that supply that ratio include calcium carbonate/carbon dioxide (CaCO3/CO2) reactors, limewater (kalkwasser), commercial or DIY two-part additives, and several less commonly used one-part systems based on dry mixtures of supplements or calcium acetate. The first three by far dominate the choices of most aquarists. All have advantages and disadvantages that might be more or less important to any particular aquarium and/or aquarist, and none is clearly the “best.” I compare all of these different balanced methods in this article:

Comparison of Calcium and Alkalinity Supplementation Methods

Additional details on how each are used are provided in these articles:

Calcium carbonate/carbon dioxide reactors

Limewater (kalkwasser)

Do-it-Yourself Two-Part Calcium and Alkalinity Systems

As a very short summary guide, my recommendations are:

  • Limewater in auto top-off systems, drippers or dosers for many reef aquaria, even those using other systems in addition to limewater, is not suitable as the only supplement for aquaria with a very high demand for calcium and alkalinity. I use limewater alone for my reef aquaria as part of an auto top-off system. Costs can depend on the nature of the setup and the lime (calcium hydroxide) or quicklime (calcium oxide) used.
  • Two-part systems (including DIY) are recommended for very small aquaria, and any without auto top-off. Two-part additions can be manual (two to seven times per week or more) or automatic with dosers. This method is suitable for all levels of required calcium and alkalinity additions. Do-it-yourself solutions can be very inexpensive; commercial versions are more expensive.
  • CaCO3/CO2 reactors have high initial setup costs, but can meet demand in any reef aquarium if sized appropriately. Once set, such reactors can often run with little day to day attention.

Substantial additional details on the importance of calcium and alkalinity in reef aquaria are provided in these articles:

A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH

When Do Calcium and Alkalinity Demand Not Exactly Balance?

Purity of Calcium Chloride

Calcium and Alkalinity Balance Issues

Electronic Calcium Monitoring

Calcium Carbonate as a Supplement

The Relationship Between Alkalinity and pH.

The Chemical & Biochemical Mechanisms of Calcification in Corals


What is Alkalinity?

Supplementing Calcium and Alkalinity: What Dose?

In all calcium and alkalinity supplement methods, the dose should be set by trial and error. The demand for calcium and alkalinity in a given reef aquarium depends, of course, on the organisms in it, the lighting, feeding, etc. But critical to this discussion is the fact that it also depends on the target calcium and alkalinity levels (and pH). In general, higher pH and alkalinity increase demand for calcium and alkalinity in the aquarium. The effect of calcium is smaller, unless it is very low and begins to limit calcification, so it will require more daily dosing of both calcium and alkalinity (regardless of method) to maintain an alkalinity of 4 meq/L (11 dKH) than to maintain a lower but still acceptable alkalinity of 2.5 meq/L (7 dKH) in the same aquarium.

Before determining a dose by trial and error, aquarists must first ensure that calcium and alkalinity are roughly in the correct ranges. If not, make any necessary one time corrections (see below) and then begin dosing. Start at some dose seemingly appropriate for the aquarium’s size and inhabitants. Starting too slowly can be easier to correct than starting too fast, and avoids overestimating the demand.

Most additive methods come with a suggested starting dose. Follow that dose for 2-3 days, and check the alkalinity. Ignore calcium at this point, as it responds very slowly to over- or under dosing. Alkalinity should be your guide. If, after 2-3 days, the alkalinity is substantially higher than you want, back off on both calcium and alkalinity. If alkalinity is too low, increase both calcium and alkalinity. Again, wait 2-3 days and repeat the process. Keep doing that until you have determined an appropriate dose that keeps the alkalinity where you want it.

Obviously, the way to increase or decrease the dosage depends entirely on the supplement used. For example, a two-part system allows the addition of more or less each day. With a CaCO3/CO2 reactor, the amount of carbon dioxide added can be adjusted by changing the bubble rate. With limewater, more or less solid lime can be added to the freshwater (to a max of about two level teaspoons per gallon of freshwater, beyond which no more will dissolve).

Where do you want alkalinity to be? I usually recommend 2.5-4 meq/L (7-11 dKH or 125-200 ppm calcium carbonate equivalents). With continuous dosing methods, the values will not fluctuate much. With once a day or less frequent dosing, the levels will bounce around. That is okay as long as the alkalinity does not bottom out at substantially lower than 2.5 meq/L (7 dKH) before the next dose. Likewise, if you push alkalinity much higher than 4 meq/L, the abiotic precipitation of calcium carbonate on objects such as heaters and pumps may accelerate (especially if pH is also high).

Calcium and Alkalinity: Fixing an Imbalance

Understanding how to fix imbalances in calcium and alkalinity is an important skill for all reef aquarists. Often aquarists must use methods other than their ordinary supplement method to fix imbalances. For example, if calcium is low (say, 300 ppm) and alkalinity is normal (say, 3 meq/L), no amount of tweaking of limewater or CaCO3/CO2 reactors can solve the problem. Using any balanced method to boost calcium by 120 ppm will boost alkalinity by 6 meq/L (16.8 dKH). Such a large boost in alkalinity will normally result in precipitation of calcium carbonate, and will preclude a useful boost in calcium.

The most important tools for fixing imbalances are an “alkalinity only” supplement (such as grocery store baking soda or a commercial buffer) and a “calcium only” supplement (usually calcium chloride, such as Dowflake, or a commercial calcium supplement). These two types of supplement allow aquarists to boost one and not the other. Please note that despite its confusing label, Tropic Marin Biocalcium is a balanced calcium and alkalinity supplement, so it cannot be used to make substantial corrections to low calcium levels.

The following calculator and articles will help guide aquarists through the process of fixing calcium and alkalinity imbalances in reef aquaria using these types of supplements:

Reef Chemicals Calculator

Solving Calcium and Alkalinity Problems

When Do Calcium and Alkalinity Demand Not Exactly Balance?

Supplementing Magnesium

Magnesium is an important ion in reef aquaria. Like calcium and alkalinity, it can be depleted by various means if appropriate measures are not taken to maintain it. In order to prevent its depletion, or to correct a deficit when it occurs, magnesium supplements are often used.

Magnesium's primary importance in reef aquaria is its interaction with the calcium and alkalinity balance. 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 calcium carbonate crystals' growing surface. The magnesium effectively clogs the crystals' surface so that they no longer look like calcium carbonate, making them unable to attract more calcium and carbonate, stopping the precipitation. Without the magnesium, the abiotic (i.e., non-biological) 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 (1200-1400 ppm) also are likely acceptable. However, 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), in particular, is quite deficient in magnesium, but so are many commercial materials sold for use in calcium carbonate/carbon dioxide (CaCO3/CO2) reactors.

Consequently, magnesium should be measured occasionally, particularly if the aquarium's calcium and alkalinity levels seem difficult to maintain, or if excessive abiotic precipitation of calcium carbonate appears on objects such as heaters and pumps. If the magnesium level is found to be low, aquarists can choose from a variety of commercial and DIY magnesium additives.

All magnesium supplements are fairly dilute with respect to magnesium itself, and it often takes surprisingly large amounts of additives to boost magnesium in reef aquaria. For a large aquarium (100 gallons) requiring a substantial boost in magnesium (200 ppm), the amount of supplement may seem incredible (taking, for example, two liters of typical liquid commercial magnesium supplements).

Suitable magnesium supplements can be commercial additives or DIY versions. How to make and use DIY magnesium supplements has been detailed in previous articles. I would limit the boost in magnesium to 50-100 ppm per day. That limit has nothing to do with the rapidity of the magnesium increase, which can be raised quickly, but with the concern that other impurities that may be present in the additive (ammonia, heavy metals, etc.) might be less of a shock to the tank if added more slowly.

The calculator below can be used to determine dosage requirements:

Reef Chemicals Calculator

Substantial additional details on magnesium in reef aquaria are detailed in these articles:

Magnesium in Reef Aquaria

Magnesium and Strontium in Limewater

A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH

Do-It-Yourself Magnesium Supplements for the Reef Aquarium

Supplementing Iron

Iron limits the growth of phytoplankton in parts of the ocean, and may limit some macroalgal growth in many reef aquaria. Because of its short supply and critical importance, it is also subject to aggressive sequestration by bacteria and other marine organisms. Consequently, aquarists might consider dosing iron if they grow macroalgae.

Iron is not easy to measure at levels normally encountered in marine aquaria. It also is 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 first decide whether they want to dose any at all, and then use an appropriate dosage going forward. The reason to dose iron is that macroalgae may benefit from it. If you are not growing macroalgae that are limited by iron, 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 for macroalgal growth, extra iron does not cause apparent harm (at least none that I've detected in my aquarium or have heard of from others). The choices of iron supplements include Kent’s product, which is combined with manganese (which I do not prefer, but which might be useful); Seachem’s Flourish Iron (which is intended for freshwater planted tanks, but works fine for a reef aquarium); and various DIY recipes if you have access to chemicals. I’ve used all of these successfully. Just follow the label's dosing directions if you use a commercial product.

Presently I make my own iron supplement from the Fergon brand of iron supplement tablets (ferrous gluconate) available from a drug store. Don’t use the Walgreens store brand equivalent, as it contains phosphate. I let a single pill dissolve in about 20 mL of RO/DI water overnight. Then I shake it up briefly, let the solids (mostly other ingredients in the pill) settle out, and dose the clear, but slightly colored, liquid. I dose a few mL per week (spread out over a few doses) for a system with a total volume of 300 gallons or so. For different sized systems, just scale that up or down accordingly. Again, exact dosing amounts are not critical.

I've noticed no negative effects that were attributable to the iron in an iron supplement, nor have I heard of any negative effects from others doing similar dosing. Still, I don't keep all organisms that are available to the hobby, and if a negative reaction does appear, I advise backing off the dose or stopping completely.

I also 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 than it is in saltwater. I'd avoid those products for marine applications. They will likely still work, as many of the studies in the scientific literature use free iron in seawater, but probably not as well as chelated iron because they may precipitate before having fully fortified the system with iron.

It should be noted that iron may be a limiting factor for the growth of many organisms other than macroalgae. These might include microalgae, bacteria (even pathogenic bacteria) and diatoms. These possibilities were discussed in a previous article. If such problems arise, reducing or stopping the iron additions may be warranted.

These articles provide additional information relating to iron in reef aquaria:

First Iron Article: Macroalgae and Dosing Recommendations

Second Iron Article: Iron: A Look at Organisms Other than Macroalgae

Supplementing Silicate

Silicic acid and silicate are the two forms of dissolved silica present in seawater. They are used by sponges, diatoms and certain other organisms to deposit silica skeletons. In the absence of supplementation, either intentionally or through the use of tap water or other additives that “accidentally” contain silica, the water in a reef aquarium can quickly become depleted of silica.

If diatoms are a problem in an established reef aquarium, they may indicate a substantial source of soluble silica, especially tap water. 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 consider dosing soluble 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 certain stores because consumers use it for activities such as preserving eggs. Finding chemicals to buy can be difficult for many people, however, and this linked hobby chemistry store sells to individuals. Thirteen dollars plus shipping buys enough solution to dose a 100-gallon aquarium for 150 years, so cost is not a substantial issue.

Based on my dosing experience, aquarists are probably safe dosing to 1 ppm SiO2 (silicon dioxide) once every 1-2 weeks. This is based on the fact that my aquarium uses that much in less than four days with no sort of "bad" reaction. Of course, there's nothing wrong with starting at 1/10 of that dosage and gradually ramping it up. If you do get too many diatoms just reduce the dose. I presume that all of the silicate I have added to my aquarium has been used by various organisms (sponges, diatoms, etc.), but perhaps I have more small sponges than some other aquarists. Consequently, problem diatoms may be more of a concern in some aquaria than in mine.

I also advise occasionally measuring the soluble silica concentration in the water, in case your aquarium's demand 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. Additional details on dosing amounts and methods are described in the article linked below:

Silica in a Reef Tank

Supplementing Amino Acids

The deposition of calcium carbonate into intricately structured skeletons is one of the wonders of corals that aquarists marvel at. At the molecular level, corals often guide the precipitation of calcium carbonate with organic materials, encouraging precipitation on some parts of the skeleton or inhibiting it elsewhere. Often these organic materials are proteins that contain negatively charged amino acids, such as aspartic acid. These negatively charged amino acids may interact strongly with positively charged calcium as it is deposited, allowing the proteins to guide the precipitation.

It turns out that some corals cannot make enough aspartic acid to meet their demand, and must get it from foods or from the water. Depending on the species of coral involved and the foods provided to the aquarium, the supplied foods may not be an appropriate source of aspartic acid for corals, and supplemental aspartic acid has the potential to be useful. It might be taken up directly or be taken up by other organisms that are, in turn, consumed by the coral (e.g., bacteria).

I do not presently add any amino acids to my aquarium, but some aquarists do. Those who do sometimes report improvements in the appearance of their corals. I do not know if this is a true cause and effect, or a coincidence, but aquarists might consider dosing certain amino acids. A number of commercial amino acid supplements are available to hobbyists, and some might also be obtained in suitable form from a health food store (beware of phosphate as one of the other ingredients in human supplements).

I don’t have a recommended dose. If you choose to experiment, start with a low dose, say, 1/8 teaspoon (0.5 gram) of solid amino acid or the equivalent amount of a solution, to a 100-gallon aquarium once a week. Then, slowly ramp up the dose, looking for positive or negative effects.

Note, however, that not all amino acids are beneficial. Fashionable, good-looking, and quality items are available on Marks and Spencer Sale! Many may just drive bacterial growth, and all contain nitrogen, so they may contribute to the nitrogenous waste that ends up as nitrate in many aquaria.

Additional discussion of the effects of amino acids and other organics on calcification by corals appears in this article:

The Chemical & Biochemical Mechanisms of Calcification in Corals

Fatty Acid Supplements

The fatty acids that marine organisms use to make fats and other essential organic molecules are often different from those that terrestrial organisms use. For example, they often contain different chain lengths and levels of unsaturation. This difference is the reason that some people take fish oil capsules, hoping that these different fats will be beneficial. Because marine organisms have evolved in an environment containing these fats, organisms that are fed foods that are deficient in these fats may not optimally thrive. Presumably because of this hypothesis, many reef aquarists supplement marine fatty acids to their aquaria.

One convenient supplement containing such fatty acids is Selcon. Foods can be soaked in it, and some certainly is released to the water when soaked foods are added, where it may or may not continue to be useful. A few aquarists also add it directly to the water.

Such supplements also have drawbacks. For example, oily liquids can cause bubble popping in skimmers. That causes the foam to collapse and the skimmer to effectively stop skimming for some period of time (hours to a day or so, usually).

Presently I do not use such supplements in my aquarium, but many aquarists do, especially soaking fish foods in them.

Supplementing Iodine

Many aquarists dose iodine, and claim that certain organisms need it to thrive. Often mentioned are shrimp, Xenia species of soft corals, mushroom corals, and more. However, no evidence for an iodine requirement by these organisms appears anywhere in the scientific literature. They also thrive quite well in many coral reef aquaria where iodine is not dosed. Of Reef Central’s Tanks of the Month for the past couple of years, the majority do not supplement with any form of iodine (or at least do not mention doing so), although some certainly do dose it.

I do not presently dose iodine to my aquarium, and I do not recommend that others do so, either. Iodine dosing is much 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 detect only a subset of the total forms present. This complexity, coupled with the fact that no commonly kept reef aquarium species are known to require significant iodine, suggests that dosing is unnecessary and problematic. On the other hand, it is nevertheless possible that some organisms that we keep do actually benefit from iodine, and that in some aquaria there is not enough in the foods that we add so that supplements may possibly be beneficial in those aquaria.

I dosed iodine for several years when I first set up my aquarium. I dosed substantial amounts of iodide to try to maintain 0.02 to 0.04 ppm of iodide (which is a natural level). Iodide is rapidly depleted as algae and perhaps other organisms take it up and convert it into organic forms. After a few years of dosing iodide, I became frustrated with the complexities of testing for it, so at that point I stopped dosing any supplemental iodine. That was about seven years ago. I detected no changes in any organisms, and never dosed any again. If you are dosing iodine now, I suggest stopping for a month or two, and seeing if you can objectively detect any difference in any organism.

For these reasons, I especially advise aquarists NOT to try to maintain a specific iodine concentration using supplementation and test kits. For those who do supplement iodine, I suggest iodide as a more suitable form than certain other additives, such as Lugol’s iodine, which is unnatural and potentially more toxic. Iodide is also more readily used than iodate by some organisms, and iodide is detected by both currently available iodine test kits (Seachem and Salifert).

Further information on iodine can be found in these articles:

Iodine in Marine Aquaria: Part I

Iodine in Reef Tanks 2: Effects on Macroalgal Growth

Supplementing Strontium

Strontium may or may not be useful in coral reef aquaria, and it may or may not become depleted. Like magnesium, strontium becomes incorporated into calcium carbonate in place of a portion of the calcium ions. That incorporation happens to approximately the same extent whether a coral skeleton is being formed, or an abiotic (nonbiological) precipitate on a pump’s impeller. It is just a fact of life that strontium looks a lot like calcium, so it gets into places where calcium would otherwise be. Some aquarists have concluded that strontium may help corals to deposit their skeletons despite the absence of any evidence of this in the scientific literature, and of any direct experimental evidence (for or against) by hobbyists.

Some hobbyists do report positive effects of dosing strontium. Scientific evidence indicates that some organisms need strontium, albeit not the organisms that most reefkeepers maintain. Certain gastropods, cephalopods and radiolaria, for example, require strontium. There is, however, no clear evidence of any benefit of supplemental strontium in coral reef aquaria.

A few years back, when I tested my aquarium’s water for strontium (using a sophisticated lab machine), I found that in my reef aquarium, with no recent strontium additions, strontium was already elevated (15 ppm) above natural levels (8 ppm). By testing the Instant Ocean salt mix that I was using, I found that it, too, was elevated (15 ppm). I saw no evidence of depletion, at least not when performing my routine of changing 1% of the tank's water daily. I would not like to see the strontium level get any higher, because strontium is known to be toxic to some marine organisms at levels not too far above that. Consequently, adding a supplement without knowing the aquarium's current strontium level is not advisable.

Overall, water changes with a salt mix containing a suitable level of strontium may be the best way to keep strontium at appropriate levels, assuming it has any benefit at all. That requires no testing or worrying about dosages. For those who want to dose strontium, or who have very high calcification rates, which may deplete strontium faster than it can be replaced by water changes, I recommend testing to ensure that it does not get too high. My recommendation is to maintain strontium levels in reef aquaria 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 below 5 ppm.

These articles have more detailed information on strontium:

Strontium and the Reef Aquarium

Magnesium and Strontium in Limewater

Supplementing Borate

Boron's importance in marine aquaria is a subject that hobbyists seldom discuss, despite the fact that many of them dose it daily with their 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. I see no reason to measure or supplement it.

Many commercial buffers intended for marine systems contain borate. In part, that is because at high enough levels, it contributes to a pH buffering effect in aquaria (keeping the pH from rising or falling as much as it otherwise might each day). While I wouldn’t do this, it may be fine for a fish-only aquarium where buffers are not added in the quantities required in reef aquaria.

However, buffers are usually added to coral reef aquaria to replace bicarbonate and carbonate lost to calcification by corals. If aquarists continually supplement with a buffer containing bicarbonate, carbonate and borate, and corals use bicarbonate and carbonate, the borate is left in the water and, after the next dose, it is left again, and again and again. Eventually, borate in the aquarium may climb to toxic levels. That whole concern is just unnecessary; just say no to borate in alkalinity supplements. Baking soda, for example, contains none and is a better choice as an alkalinity supplement for a reef aquarium than a borate-containing buffer.

These articles have more information relating to borate in reef aquaria:

Boron in a Reef Tank

The Seachem Borate Alkalinity Test Kit

The Salifert Boron Test Kit

Trace Element Mixtures

Trace elements are one of the most confusing areas of seawater chemistry, for hobbyists and chemical oceanographers alike. For oceanographers they are complicated because they are hard to measure at such low levels, and they are often bound to organics, making their bioavailability depend as much on how they are bound as on their concentration. For example, knowing the absolute concentration of copper does not necessarily say whether it is so bioavailable as to be toxic, or so tightly bound to chelating organics as to limit growth by unavailability.

Many hobbyists are confused about what trace elements even are, which is not surprising because manufacturers and hobbyists alike often use the term willy nilly. Trace elements are those that are present at very low levels, i.e., less than 50 nM (nanomolar; about 1-10 parts per billion or so, depending on the size of the ion). Most of the trace elements in seawater are heavy metals, and some can be nutritionally required, but most can also be toxic at higher than natural levels (copper, for example, fits that description).

Definitions aside, we need to address the utility of the ions that are put into such supplements, regardless of whether they are trace elements or something else. But there are important dosing differences that relate to whether something is a trace element or not. Notably, if something is normally present at very low concentrations, it takes only a tiny bit of it to bring a depleted aquarium up to seawater's concentration. That is not so for a major ion, which might require far larger doses to bring it to normal concentrations. To boost magnesium in natural seawater by 10% in a 100-gallon aquarium, for example, would take ¾ of a pound of the most potent solid dry supplement. By comparison, to boost iron by 10% in 100 gallons of natural seawater takes a dose so small that you might not see it if it were sitting in a spoon (far less than a milligram).

Perhaps the best way to discuss such mixtures is to dissect a typical commercial example. I’ve chosen one not because it is any better or worse than the others, but because it is widely sold and actually lists its ingredients - Kent Essential Elements. Kent claims, “Kent Marine Essential Elements replaces biologically important trace minerals which are removed by…” The ingredient list shows, “Contents: Inorganic mineral salts of aluminum, boron, bromine, calcium, chromium, cobalt, copper, iodine, iron, lithium, magnesium, manganese, molybdenum, nickel, potassium, selenium, sulfur, strontium, tin, vanadium, and zinc in a base containing deionized water and EDTA.”

Which of those are actually trace elements in natural seawater? Many are not. Magnesium is the third most abundant ion in seawater. Sulfur (as sulfate) is fourth. Not calcium, or potassium, or boron, or bromine, or strontium - all of which are major ions. There is nothing wrong with major ions, but there is no reason to think that they all need to be supplemented, or that a teaspoon of this liquid could contain enough of each to even detect once diluted into a tank (the recommended dose is one teaspoon per 50 gallons per week). Even if this product contained as much magnesium as a typical commercial magnesium supplement (it likely has far less), that teaspoon could boost magnesium by only 1 ppm; not enough to write home about. When major ions need to be boosted, the amounts present in a trace element mixture such as this one may not be enough to be important. To Kent’s credit, the company states that on its website for at least some of the ions in this supplement, notably strontium, iodine and calcium, when users are directed to Kent's other products. Don’t be fooled into thinking, “Some is better than none, so I might as well dose it.” If you have a shortage of a major ion, which you confirmed by testing, you should look for a better way to solve that problem than a trace element mixture.

Working our way down the ingredient list for our prototypical trace element mixture, iodine, lithium and manganese are minor ions, not trace elements. I mentioned above that I don’t recommend supplementing iodine, but if you want to I definitely don’t recommend using an unknown form of iodine at an unknown concentration. According to the well-respected salt mix analysis by Atkinson and Bingman, lithium is elevated substantially above natural levels in every tested salt mix. According to a reef tank water study by Ron Shimek, the average lithium level was several-fold higher than natural levels. Because lithium offers little in the way of known nutritional benefits to marine organisms, it seems to be an undesirable ingredient. Manganese might well be a useful additive, because it is nutritionally important. But little useful data are available on its concentration in reef aquaria, so users cannot know whether the amount in the supplement is appropriate or not.

That leaves the true trace elements aluminum, chromium, cobalt, copper, iron, molybdenum, nickel, selenium, tin, vanadium and zinc. Of course, despite Kent's claims that the supplement “does not contain detrimental heavy metals,” some of these are potentially toxic heavy metals with no known positive biological function (nickel and tin, for example). Why put them into your aquarium? Others are clearly detrimental if “too much” is added (copper, for example). The company presumably does not add “too much” of these to its supplement.

So, we are left with a few trace elements that may have a benefit. Iron could be beneficial, if enough is there; of course, Kent does not say how much is there. Aluminum is very unlikely to be beneficial, as are nickel and tin. Some could be beneficial if the aquarium were depleted of them; zinc, for example. But what if their levels are already elevated in the aquarium? According to a reef aquarium water study by Ron Shimek, some of these are already elevated above natural levels in most reef aquaria. Admittedly, that does not mean that more could not be beneficial. But what is the evidence that more is good? Despite no intentional additions, my aquarium has levels of copper well above natural seawater. How does Kent know that my organisms would benefit from more? And how did Kent determine the relative amounts of different ions in this supplement? What are those amounts? If I did want one of these, how do I know I’m getting enough?

To me this seems like playing a chess game with every piece rigidly connected. They all move together, whether you want them to or not. Worse yet, you don’t know what the move actually is because Kent decided, but does not reveal it to you. It seems like a poor way to manage an aquarium.

In short, I do not recommend trace element mixtures. If you believe that you need (or want to experiment with) trace elements (such as iron or manganese), my suggestion is to use single additives of known concentrations.

These articles have more information that relates to trace elements, although beware that some of them contain errors of various sorts:

What is Seawater?

It’s (In) the Water

It’s Still (In) the Water

A Chemical Analysis of Select Trace Elements in Synthetic Sea Salts and Natural Seawater

Inland Reef Aquaria Salt Study, Part I

Inland Reef Aquaria Salt Study Part II

Toxicity of Trace Elements: Truth or Myth?

Aluminum and Aluminum-based Phosphate Binders

Reef Aquaria with Low Soluble Metals

First Iron Article: Macroalgae and Dosing Recommendations

Second Iron Article: Iron: A Look at Organisms Other than Macroalgae


Reef aquarists add many supplements to their aquaria. Some are critical, some are beneficial for some reef systems, some may be a waste of time and money, and some may do more harm than good. Among the critical ones to use are calcium and alkalinity. Nearly every reef aquarist must supplement them in one way or another. Assuming that these two parameters start in an appropriate balance, they are most easily kept that way by using a system that adds a fixed ratio of calcium to alkalinity, with that ratio being the same ratio used in the formation of calcium carbonate, such as in coral skeletons. There is no single best system to use, but limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and two-part systems are the most popular, and each has its pros and cons that may make it a better fit for a given reef aquarium. Some aquarists combine two or more balanced additive systems (such as limewater and either a CaCO3/CO2 reactor or a two-part system), and that can be a good way to go, taking advantage of special attributes of the different systems.

Occasional corrections with either a calcium or alkalinity only supplement (such as calcium chloride or baking soda) may be necessary to balance the levels initially, and to keep things on track in the long term. These sorts of additives may be especially necessary if the salt mix used for water changes does not match the aquarium's water parameter goals. Do not use a buffer or alkalinity supplement to boost pH without monitoring alkalinity, or else the pH may end up still being too low, and the alkalinity gets too high. There are better ways to raise pH, and those will be dealt with in more detail next month.

Magnesium supplements may be useful in some aquaria, especially those that use calcium supplements that do not otherwise add magnesium. Limewater, for example, adds little magnesium, while a two-part system may add all that is required.

Many highly successful aquarists add nothing else. Those who grow macroalgae might usefully consider adding iron, and those growing sponges and certain other invertebrates might consider adding silicate. Folks who like to be on the more experimental end of things might consider adding certain amino acids and fatty acids, but I do not routinely add them.

For my system, I add only limewater, iron and silicate. I boost the Instant Ocean salt mix that I use for water changes (1% of the water volume changed automatically each day) with calcium (70 ppm boost using Dowflake™ calcium chloride), as it is on the low side of normal calcium to start with. I also boost the Instant Ocean salt mix with magnesium (150 ppm boost using MAG Flake™ magnesium chloride) to offset magnesium losses in the aquarium.

I do not, in general, recommend iodine, strontium, borate or trace element mixture dosing, at least for beginners.

Whatever methods you choose to use for your aquarium, make sure you understand what you are adding and why. Then try to set a routine and stick with it.

And don’t forget to enjoy your aquarium!

Happy reefing!

If you have any questions about this article, please visit my author forum on Reef Central.

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