Phosphate In The Reef Aquarium: By Randy Holmes-Farley

 

The phosphorus atom is one of living matter’s basic building blocks. It is present in every living creature and in the water of every reef aquarium. Unfortunately, it is often present in excess in reef aquaria and that excess has the potential to cause at least two substantial problems for reefkeepers. The first is that phosphate is often a limiting nutrient for algae growth, so when elevated it can permit excessive growth of undesirable algae (potentially including the zooxanthellae inside of corals, turning them brown). The second is that it can directly inhibit calcification by some corals and coralline algae. Because most reefkeepers don’t want any of these things to happen, they strive to keep phosphorus levels under control. Fortunately, there are several effective ways to accomplish this.

Phosphorus exists in two primary forms in seawater: as inorganic phosphate, especially orthophosphate, and as organophosphate. Orthophosphate is readily taken up by algae and actively inhibits calcification. The organic forms may or may not be available to organisms such as algae. Aquarists can readily test for inorganic orthophosphate using a standard aquarium phosphate test kit, but testing for organic phosphorus compounds with a kit is considerably more tedious. Moreover, if there is an algae problem, then the algae may be consuming the orthophosphate as fast as it enters the water, thereby masking the issue. Consequently, many reef aquarists may not recognize that they have a phosphate problem, only that they have an algae problem.

Phosphate in Seawater

The “simplest” form of phosphorus in seawater is inorganic orthophosphate (sometimes called Pi by biologists). It consists of a central phosphorus atom surrounded by four oxygen atoms in a tetrahedron (Figures 1 and 2). Three of these oxygen atoms can either have an attached hydrogen atom or carry a negative charge (Figure 2). The ratio of these different forms depends on the pH in seawater. At pH 8.1, seawater contains 0.5% H₂PO₄^–, 79% HPO₄^— and 20% PO₄^—. At higher pH the equilibrium shifts toward more PO₄^— and less HPO₄^—. For a variety of reasons, especially including the ion pairing and consequent stabilization of PO₄^— by calcium and magnesium, there is far more PO₄^— in seawater than in freshwater at the same pH. This shift in phosphate species distribution with pH may seem esoteric, but it actually has important implications for such things as the binding of phosphate to calcium carbonate rock and sand, because the different forms bind to different extents.

Figure 1. The three dimensional structure of inorganic orthophosphate, shown in a fully protonated form. It is comprised of a central phosphorus atom (purple) and four oxygen atoms (red) arranged in a tetrahedron. Three of the oxygen atoms are shown with an attached proton (white).

Figure 2. The structure of inorganic orthophosphate, with a central phosphorus atom (purple) and four oxygen atoms arranged in a tetrahedron. Three of the oxygen atoms can either have an attached proton (green) or be present with a negative charge on the oxygen atom (red). The amount present in each form in seawater varies with pH, as indicated.

The orthophosphate concentration in the ocean varies greatly from place to place and also with depth and time of day. Surface waters are greatly depleted in phosphate relative to deeper waters, due to biological activities that sequester phosphate in organisms. Typical ocean surface phosphate concentrations are very low by reefkeeping standards, sometimes as low as 0.005 ppm.¹

At concentrations below about 0.03 ppm, the growth rate of many phytoplankton species depends on the phosphate concentration (assuming that something else, such as nitrogen or iron, is not limiting their growth). Above this level, many organisms’ growth rate is independent of phosphate concentration.¹ Consequently, to deter algal growth by controlling phosphate, aquarists need to keep the phosphate levels quite low.

Other Forms of Inorganic Phosphate

Phosphorus also can take other inorganic forms, such as the polyphosphates, which are rings and chains of phosphate ions strung together by P-O-P bonds. While these are usually insignificant in natural seawater, they can be present in various solutions that are added to reef aquaria. There are many of these compounds, but most will likely break down into orthophosphate when added to a reef aquarium. Polyphosphates are used industrially to bind metals, such as in some laundry detergents. In that application, they form soluble complexes with calcium and magnesium, softening the water and enhancing cleaning action. The amount of phosphate entering natural waterways from laundry detergents, however, is high enough that algae blooms sometimes result, and the practice of using phosphate in detergent is now illegal in many places.

Organic Phosphate

In seawater, organic phosphorus compounds are far more varied and complex than inorganic phosphate. Many common biochemicals contain phosphorus and every living cell contains a wide variety of them. Molecules such as DNA (deoxyribonucleic acid), ATP (adenosine triphosphate), phospholipids (e.g., lecithin) and many proteins contain phosphate groups. In these molecules, the basic phosphate structure is covalently attached to the remainder of the organic molecule through one or more phosphate ester bonds to a carbon atom.

These bonds are stable for some period of time in water, but eventually break down to release inorganic orthophosphate from the molecule’s organic part, a process that can be sped up through the action of enzymes in a reef aquarium. Many of these organic phosphate compounds will be readily removed from an aquarium by skimming. Export of organic phosphates is likely the major way that skimming can reduce inorganic orthophosphate levels in an aquarium. Orthophosphate ions are not significantly removed by skimming because they do not adsorb onto an air/water interface. but many organic phosphates can be removed by skimming or other organic export methods. These other methods include granular activated carbon (GAC) and polymeric binders such as Purigen or a Polyfilter, which remove the ions before they are broken down into inorganic orthophosphate.

An important point about organic phosphates is that most of them are not readily bound by inorganic phosphate-binding materials used in the aquarium hobby. Consequently, while these products may do a fine job of reducing inorganic orthophosphate, they may not substantially reduce organic phosphates.

A final point is that organic phosphates are not detected by most test kits designed for hobbyists. Those that do detect organic phosphates, e.g., Hach PO-24, break the phosphate off the organic compound, thereby converting it into inorganic orthophosphate prior to testing. These kits are tedious to use and expensive, however, so they’re not for every hobbyist. Indeed, I’ve never used one.

Testing by ICP (inductively coupled plasma) by a commercial enterprise such as Triton or ENC Labs will give a value for total phosphorus, including both organic and inorganic forms. Depending on how the sample is prepared and processed, the organic forms may be individual molecules dissolved in the water, or larger aggregates, all the way up to and including whole bacteria (although Triton says these are removed before testing). Now that larger numbers of reef aquarists are getting ICP testing done, it does not appear that total phosphorus/phosphate is noticeably higher than what those same reefers are getting with test kits, so the organic forms may often be a minor contributor to total phosphate in many reef aquaria.

Organics in seawater are often measured in terms of their nitrogen content, such as dissolved organic nitrogen (DON) and particulate organic nitrogen (PON). The same is true for phosphorus, using the terms dissolved organic phosphorus (DOP) and particulate organic phosphorus (POP). Table 1 shows the relative concentrations of C, N and P in typical dissolved organic material found in seawater.¹ In dissolved organic material, nitrogen is about tenfold less prevalent than carbon, and phosphorus is several hundredfold lower in concentration than carbon.

Phosphate Sources in Reef Aquaria

Organic phosphorus compounds, as well as orthophosphate, are so prevalent in biological systems that any natural food necessarily contains significant concentrations. Not only can organic material be taken up directly to provide carbon, nitrogen and phosphorus, it can be broken down by organisms and released as inorganic nutrients, such as orthophosphate, ammonia, nitrite and nitrate.

Foods are nearly always the primary source of phosphate in reef aquaria, and rarely does a reefer need to look further when tracking down a phosphate problem. Even the small amounts that might come in with top off water (at, say 0.05 ppm phosphate) or on granular activated carbon are usually tens to hundreds of times less than what comes in with feeding, despite tests that purport to show that these are a significant sources. Don’t agonize over ANY other source, unless you may be using unpurified tap water. I discuss the relative importance of different sources of phosphate, including different types and brands of foods, in great detail in this linked article.

The metabolic breakdown scheme for typical organic materials in phytoplankton¹ is shown below:

(CH₂O)₁₀₆(NH₃)₁₆(H₃PO₄) + 138 O₂ → 106 CO₂ + 122 H₂O + 19 H⁺ + PO₄^— + 16 NO₃^–

organic + oxygen → carbon dioxide + water + hydrogen ion + phosphate + nitrate

Flake fish food is typically about 1% phosphorus (3% phosphate equivalent) by weight. Many products have phosphorus-content data on their labels. Consequently, if 5 grams of flake food is added to a 100-gallon aquarium, there is the potential for the inorganic orthophosphate level to be raised by 0.4 ppm in that SINGLE FEEDING! That fact can be a significant issue for reefkeepers.

What do we do with all of that phosphorus? If the food is completely converted into tissue mass, then there will be no excess phosphate. But much of the food that any heterotrophic organism consumes goes to provide energy, leaving a residue of CO₂ (carbon dioxide), phosphate, and a variety of nitrogen-containing compounds (ammonia, nitrite, nitrate), as shown above. A fish, whether it is an adult or a growing juvenile, consequently excretes almost all of the phosphorus that it takes in with its food as phosphate in its waste. Of course, overfeeding will result in more phosphate delivery than will reduced feeding levels.

I do not mean to implicate flake food as a worse culprit than frozen or fresh foods. They all contain large amounts of phosphate, but the latter often do not provide analyses on their label. Unfortunately, many types of seafood available at the grocery store have various inorganic phosphate salts intentionally added to them as preservatives. These foods include canned and frozen seafood, as evidenced by their label, and even some fresh seafood. In these cases, rinsing the food before using it may help to reduce the phosphate load it adds to the aquarium, but will definitely not eliminate the concern for phosphate coming in with the food.

Finally, tap water can also be a significant source of phosphate. The tap water that the Massachusetts Water Resources Authority supplies to me is acceptably low in phosphate, or at least it was the last time I measured it. In other water supplies, however, phosphate levels can be too high for reefkeeping. In 2013 New York City officials reported that water samples showed phosphate levels as high as 4 ppm, with an average of 2 ppm. I’d recommend phosphate testing of tap water to anyone with an algae problem who uses unpurified tap water, in order to ascertain whether phosphate in the water is an issue.

On the other side of this issue are those reef aquaria without fish. Because phosphorus is required for growing tissue, it is mandatory that some phosphorus source be available to corals and invertebrates growing in a reef aquarium. Finding a source is trivial if fish are in the aquarium, but in aquaria without fish, reefkeepers must somehow add phosphorus. The solution to this problem is easy: either add fish food, even though there are no fish, or add a source of phosphorus such as an aquarium plant fertilizer. If the fertilizer does not contain nitrogen, a source of that may be needed as well.

Calcification Inhibition by Phosphate

One important issue relating to elevated phosphate in reef aquaria has to do with the inhibition of calcification by phosphate and phosphate-containing organics. Phosphate is known to inhibit the precipitation of calcium carbonate from seawater.^2-4 The presence of phosphate in the water also decreases calcification in corals, such as Pocillopora damicornis⁵ and entire patch reefs.⁶ This inhibition is likely related to the presence of phosphate in the extracytoplasmic calcifying fluid (ECF), where calcification takes place in corals⁷, and on the growing crystal’s surface. Exactly how the phosphate gets into the ECF isn’t well understood.

This inhibition of calcification takes place at concentrations frequently attained in reef aquaria and may begin at levels below those detectable by hobby test kits. For example, one research group found that long-term enrichment of phosphate (0.19 ppm; maintained for three hours per day) on a natural patch reef on the Great Barrier Reef inhibited overall coral calcification by 43%.⁶ A second team found effects in several Acropora species at similar concentrations.⁸

Figure 3. The chemical structure of the organophosphate “etidronate,” shown in a fully protonated form.

Organic phosphate and phosphonate inhibitors of calcification have also been studied and probably work by a similar mechanism. Etidronate, a bisphosphonate that is used to treat osteoporosis (Figure 3), caused a 36% inhibition of calcification in Stylophora pistillata at 2 ppm, and stopped it completely (99%) at 100 ppm, while photosynthesis was not affected at these, and higher, concentrations (indicating it is not a general toxin).⁹

With all this said, however, there are a few very nice reef aquaria that have exceptionally high phosphate, up to 2 ppm. This linked article has more details. Presumably, pest algae in these aquaria are inhibited by something other than low nutrients, and iron is a likely candidate. How such systems get around the inhibition of calcification is unclear, but apparently they can.

How to Export Phosphate

Now that we know where phosphate comes from, and what impact it has, we can proceed to ask where it goes and how to maximize those export processes. Certainly, some phosphorus goes into the bodies of growing organisms, including bacteria, algae, corals, and fish. Some of these organisms stay permanently in the aquarium, and others may be removed by algae harvesting, skimming of small organisms, and even pruning of corals. These and other mechanisms are discussed in subsequent sections of this article.

Phosphate Reduction via Calcium Phosphate Precipitation

One mechanism for phosphate reduction in reef aquaria may simply be the precipitation of calcium phosphate, Ca₃(PO₄)₂. The water in many reef aquaria is supersaturated with respect to this material, as its equilibrium saturation concentration in normal seawater is only 0.002 ppm phosphate. As with CaCO₃, the precipitation of Ca₃(PO₄)₂ in seawater may be limited more by kinetic factors than by equilibrium factors, so it is impossible to say how much will precipitate under reef-aquarium conditions (without, of course, somehow determining it experimentally). This precipitation may be especially likely where calcium and high pH additives (such as limewater/kalkwasser) enter the aquarium water. The locally high pH converts much of the HPO₄^— to PO₄^—. Combined with the locally high calcium level (also from the limewater), the locally high PO₄^— level may push the supersaturation of Ca₃(PO₄)₂ to unstable levels, causing precipitation. If these calcium phosphate crystals are formed in the water column, e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and be skimmed out of the aquarium.

Many reefkeepers accept the concept that adding limewater reduces phosphate levels. This may be true, but the mechanism remains to be demonstrated. Craig Bingman has done a variety of experiments related to this hypothesis, and published them in the old Aquarium Frontiers magazine. While many aquarists may not care what the mechanism is, knowing how it occurs will help us understand the limits of this method, and how to best employ it.

One possible mechanism could be through calcium phosphate precipitation, as outlined above. A second mechanism for potential phosphate reduction when using high pH additives is the binding of phosphate to calcium carbonate surfaces. The absorption of phosphate from seawater onto aragonite is pH dependent, with the binding maximized at around pH 8.4 and with less binding occurring at lower and higher pH values. Habib Sekha (owner of Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef aquaria. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef aquaria exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many aquarists find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way this can be true is if the excess calcium and alkalinity, which such additions typically add to the aquarium, are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock). It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels; phosphate binds to these growing surfaces and becomes part of the solid precipitate.

If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can inhibit the complete removal of excess phosphate from a reef aquarium that has experienced very high phosphate levels, and may permit algae to continue to thrive despite all external phosphate sources having been cut off. In such extreme cases, removal of the substrate may even be required.

If the calcium carbonate deposits are growing, then phosphate may become buried in the growing crystal, which can act as a sink for phosphate, at least until that CaCO₃ is somehow dissolved. Additionally, if these crystals are in the water column, e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and skimmed out of the aquarium.

If phosphate binds to calcium carbonate surfaces to a significant extent in reef aquaria, then this mechanism may be attained with other high pH additive systems (such as some of the two-part additives, including Recipe #1 of my DIY system). However, this potential precipitation of phosphate on growing calcium carbonate surfaces will not be as readily attained with low pH systems, such as those using calcium carbonate/carbon dioxide reactors or those where the pH is low due to excessive atmospheric carbon dioxide, because the low pH inhibits the precipitation of excess calcium and alkalinity as calcium carbonate, as well as inhibiting the binding of phosphate to calcium carbonate.

Phosphate Uptake by Organisms: Microalgae

It’s frequently said that limiting phosphate will limit algae growth in reef aquaria. That is almost certainly true, but some species of microalgae thrive more readily under phosphate limitation than others (click here for a phosphate limitation study). Bryopsis and Debresia are, for example, two types of pest algae that reef aquarists are generally unable to completely deal with by reducing phosphate. Presumably there is a phosphate level below which it will die, but such low levels may also kill organisms we want to thrive (such as corals). Some species of microalgae can, in fact, significantly regulate their inorganic phosphate transport capabilities to deal with variable phosphate levels. Finally, organic phosphates may also need to be considered in some cases. Many organisms can enzymatically break down organic phosphates into inorganic orthophosphate prior to their absorption. Consequently, we are left with an incomplete understanding of what organisms in our aquaria use what forms and concentrations of phosphorus. Further complicating matters, our aquaria are usually greatly skewed from natural seawater in terms of other nutrients, such as nitrogen and iron, so we cannot readily extrapolate from phosphate studies in seawater to draw any conclusions about our aquaria.

Nevertheless, many aquarists are very successful at dealing with algae problems by reducing phosphate using one of the mechanisms detailed later in this article. Even if phosphate is very low (say, 0.02 ppm or less), a large algae outbreak can often be resolved by diverting the phosphate away from the algae and into some other export mechanism. As mentioned above, at inorganic phosphate concentrations below about 0.03 ppm, the growth rate of many species of phytoplankton depends on the phosphate concentration, assuming that something else, such as nitrogen or iron, is not limiting growth. Above this level, many organisms’ growth rate is independent of phosphate concentration.¹ So, to deter algae growth by controlling phosphate, aquarists need to keep the phosphate levels quite low. In a problem algae situation, 0.01 to 0.02 ppm or even lower may be an appropriate goal.

Having sufficient microalgae can keep the phosphate level below 0.02 ppm. An ATS (algal turf scrubber), for example, grows turf algae as a way of exporting nutrients, including phosphate. The same is true for macroalgae, where an adequately large amount of macroalgae can keep phosphate below 0.02 ppm, and hence the reason that many aquarists use macroalgae or ATS systems to export phosphate. Consequently, the phosphate concentration itself is an inaccurate guide to whether exporting phosphate is going to help resolve an algae problem. In fact, it almost always seems to be effective, although exporting adequate phosphate is not always easy, especially if the phosphate concentration is higher than 0.2 ppm.

Phosphate Export by Organisms: Macroalgae

As mentioned above, growing and harvesting macroalgae can be a very effective way to reduce phosphate levels (along with other nutrients) in reef aquaria. In my reef system, where I have large, lit refugia to grow the macroalgae Caulerpa racemosa and Chaetomorpha sp., these algae are clearly a significant phosphate export mechanism. Aquaria with large amounts of thriving macroalgae can avoid microalgae problems or excessive phosphate levels that might inhibit coral calcification. Whether the reduction in phosphate is the cause of the microalgae reduction is not always obvious; other nutrients can also become limiting. But to reef aquarists with a severe microalgae problem, the exact mechanism may make no difference. If rapidly growing macroalgae absorb enough phosphorus to keep the orthophosphate concentrations in the water column acceptably low, and at the same time keep microalgae under control, most reefkeepers will be satisfied.

For those interested in knowing how much phosphorus is being exported by macroalgae, this free PDF article in the journal Marine Biology has some important information. It gives the phosphorus and nitrogen content for nine different species of macroalgae, including many that reefkeepers typically maintain. For example, Caulerpa racemosa collected off Hawaii contains about 0.08% phosphorus by dry weight and 5.6% nitrogen. Harvesting 10 grams (dry weight) of this macroalgae from an aquarium would be the equivalent of removing 24 mg of phosphate from the water column. That amount is the equivalent of reducing the phosphate concentration from 0.2 ppm to 0.1 ppm in a 67-gal. aquarium. All of the other species tested gave similar results (plus or minus a factor of two). Interestingly, using the same paper’s nitrogen data, this would also be equivalent to reducing the nitrate content by 2.5 grams, or 10 ppm in that same 67-gal. aquarium.

Organic Carbon Dosing and Bacteria

A second means of exporting phosphate is bacterial growth. Such growth can be spurred by adding carbon sources to the water. Some carbon sources include sugar, acetic acid (vinegar) and ethanol (ethyl alcohol, often as vodka) or a solid material such as biopellets. Vodka and vinegar are by far the most popular as a DIY. I use vinegar. A variety of commercial systems also add carbon sources, although they rarely reveal exactly what ingredients they contain. These bacteria feed on the added carbon sources, using them as a source of energy. As they grow and multiply, they necessarily take up nitrogen and phosphorus from the water to form the many biomolecules that they contain, such as DNA, RNA, phospholipids, etc. These bacteria may then be partially removed by skimming.

The dosed organic molecules can be used by many organisms, including corals, but the main intent is to drive bacterial growth. To grow, the bacteria need a source of nitrogen and a source of phosphate, and a large portion of these they remove directly from the water. The bacteria may grow out of sight (inside live rock or sand, in refugia, in tubing). They may also grow in globs in the display tank. They have to grow somewhere. If they become unsightly, try dosing a different organic that may drive a different set of species that may grow in a different location. Too much suspended bacteria might be temporarily eliminated with a UV sterilizer, forcing more bacteria to grow on surfaces. However, I would not run a UV full time when using organic carbon dosing as I’d prefer the bacteria to remain alive and whole until they are eaten (by sponges, etc.) or skimmed out, rather than die and spill their internal contents prematurely. I’ve had bacteria seem to grow on GAC (granular activated carbon media) in a canister filter I previously used, allowing relatively easy export by rinsing the GAC once every couple of weeks.

I’ve never heard any plausible argument why dosing multiple organics at once is desirable, but many people do it and there is likely no harm in doing so. The idea that multiple organics drive a diversity of bacterial species is just speculation, and even if true, I don’t see the benefit.

The bacteria themselves can be skimmed out, or used as a food for filter feeders, or both (most people probably have both to some extent, unless they do not use a skimmer). The bacteria may grow partly in low O₂ regions (such as in sand or rock) and partly in highly oxygenated environments. Since metabolism in low-O₂ regions uses relatively more nitrate than phosphate compared with metabolism in a high-O₂ environment, the relative amounts of nitrate and phosphate reduction an aquarist observes may vary from system to system. Nitrate is always reduced to a greater extent than phosphate simply because bacteria need a lot more nitrogen than phosphorus, but metabolism of organics in low-O₂ regions may skew it even more, and sometimes can leave the aquarium with little nitrate and an excess of phosphate that the bacteria don’t “want.” In such a case, a phosphate binder might usefully export this remaining phosphate. Alternatively, some aquarists have dosed nitrate directly to the aquarium to allow the residual phosphate to be consumed. Potassium nitrate stump remover and calcium nitrate are common sources that people have used.

One potential drawback that may have played a role in some tank problems is that the bacteria that thrive when organic molecules are dosed may be benign (and appear to be in almost all cases), but might actually be pathogenic in others. That is, the added organics may enhance bacterial infections if those bacteria causing the infection, of fish and corals, are able to take up the added organics and use them to grow faster. I think this risk is low, but it may be real. If you have unexplained problems that might fit this description, and are organic carbon dosing, try not dosing for an extended period.

A second potential drawback of organic-carbon dosing is the potential for proliferation of unsightly cyanobacteria in the display tank. There are many species of cyanobacteria, and some can consume the organics we add in this method. If they become a primary consumer, then something may need to be done, such as switching to a different organic compound to dose, or reducing phosphate with a binder such as GFO (granular ferric oxide).

One additional drawback to this process, relative to doing the same thing with macroalgae, is that the bacteria consume oxygen and reduce pH as they metabolize organic compounds. Macroalgae, on the other hand, require large areas exposed to light, using costly energy and what is often most limited: real estate in the vicinity of the aquarium.

These linked articles describe vinegar and vodka dosing in more detail.

Phosphate Export by Skimming

Skimmers generally remove organic compounds, and leave behind inorganic compounds such as inorganic orthophosphate. The organic compounds removed by a skimmer collectively contain carbon, hydrogen, nitrogen, phosphorus, and sulfur, among other atoms. So skimming and exporting organics tend to have the very useful attribute of exporting these molecules before they can be broken down into phosphate, nitrate, and sulfate. Many organisms, from fish and people to bacteria, for example, take in organic materials as a source of energy and release the excess nitrogen, sulfur, and phosphorus not needed for growth. In many cases in an aquarium these excreted materials end up as phosphate, nitrate, and sulfate, either by direct excretion, as in the case of phosphate and nitrate, or as ammonia, urea, or other nitrogen-containing compounds that, through additional bacterial processing, can end up as nitrate.

Whole bacteria may also be skimmed out, and this is potentially one way that adding organic carbon (vodka) to a reef aquarium actually exports phosphorus and nitrogen). Inorganic orthophosphate itself does not adsorb onto an air/water interface, and so will not be directly skimmed out. In fact, such highly charged ions as phosphate are actually repelled by an air/water interface, where they are unable to be effectively hydrated on the side exposed to the air.

Phosphate Export Using Binding Media

Many commercial phosphate binders are used in reef aquaria. Many of these are inorganic solids that bind phosphate onto their surfaces. One common type is aluminum oxide, such as Seachem’s PhosGuard and Kent’s Phosphate Sponge. Another common type is GFO (granular ferric oxide), which generally is iron oxide hydroxide. Brands include ROWAphos, PhosBan, and Salifert Phosphate Killer). These materials primarily bind inorganic orthophosphate, but they may bind some organic materials as well, including some types of organic phosphate.

Many people (including myself) have successfully used these products, but they have attributes that some aquarists may be concerned about. The following sections of this article detail these products more fully but, in general, the inorganic binding media have the potential to partially dissolve into the aquarium water, releasing their primary components (aluminum and iron, for example), as well as impurities that may be incorporated into them. Despite claims to the contrary, these materials are also likely to reversibly bind phosphate, and so may release the phosphate back into the water under certain circumstances.

Activated carbon is not expected to bind large amounts of inorganic orthophosphate, but it does effectively bind many organic materials that contain phosphate (such as phospholipids). In general, however, if phosphate reduction is the primary goal, there are likely better ways to accomplish it than by using activated carbon.

Some organic polymers (including my own pharmaceutical products, Renagel^® and Renvela^®) are designed to bind phosphate in various forms. While some of these are sold to aquarists and claim to bind phosphate, such organic materials are not very effective at binding inorganic orthophosphate under conditions present in seawater. Such materials experience great competition for phosphate binding sites by the very large amounts of chloride (Cl^–) and sulfate (SO₄^—) in seawater. They may effectively bind organic compounds that contain phosphate, however, in a fashion similar to activated carbon.

Phosphate Export Using Binding Media: Aluminum Oxide

Aluminum oxide is the primary ingredient in several commercial phosphate binders, such as Seachem’s PhosGuard™. These materials are always white solids, although not all white phosphate binders are necessarily aluminum oxide. Phosphate binds strongly to aluminum ions exposed on the surface of aluminum oxide solids. Phosphate is believed to bind to aluminum-containing surfaces through a direct ionic interaction between one or two negatively charged oxygen ions on the phosphate with the aluminum ions (Al⁺⁺⁺) exposed on the solid surface. After exposure to the aquarium water for sufficient time to adsorb phosphate, the solids are removed and the phosphate is removed along with it. This process has been used historically in other industries as well, including phosphate binding in people, where aluminum use is no longer recommended due to toxicity concerns.

Unfortunately, aluminum oxide is not completely insoluble in seawater. I have shown experimentally that aluminum can be released from PhosGuard, and I have also shown that adding the same amount of released aluminum back into an aquarium can irritate corals, causing them to retract their polyps and otherwise shrink. That effect mirrors what some aquarists reported (prior to this test) as a side effect of using these media. Rinsing the solids before use can reduce the likelihood that small aluminum-containing particulates are released into the aquarium, but it does not prevent the solubilization of aluminum ions from the solid surfaces.

That all said, many people use aluminum oxide effectively, and many never notice any negative effects. I have used it in the past without noticing harm in my aquarium, although I have used only small amounts. Rinsing it before use and not using large amounts all at once will limit any negative impact.

Phosphate Export Using Binding Media: Granular Ferric Oxide/Hydroxide

In the past few years iron-based phosphate binding materials have become very popular among reef aquarists. These materials have been used commercially to treat drinking water (to remove arsenic, for example) and to treat wastewater (to remove a wide range of pollutants, including phosphate). They are sold to aquarists under a wide variety of different brand names, including PhosBan^®, Phosphate Killer™, and ROWA^®phos. These materials all range in color from reddish brown to nearly black. In a previous article I detailed how they function as well as some of the concerns that aquarists have had when using this material.

Even though the commercial materials appear to be reasonably large particles (Salifert claims 0.2 – 2 mm on its product label), they actually have a high internal surface area, somewhat similar to activated carbon. Consequently, apparent particle size is an unreliable means by which to gauge available surface area (though it is reliable for nonporous solids such as table salt). I have seen no measures of accessible surface area for the commercial granular ferric oxide (GFO) sold to aquarists. There are various modifications to the standard material, such as forming it into pellets (to perhaps work better in some applications such as media bags) and enclosing the GFO in a polymeric matrix (reducing the potential for breakage of the particles).

Phosphate bound to GFO surfaces is still available to the water column by exchange, so the sequestering is temporary rather than permanent. This fact is known in the literature³, and can be easily demonstrated by adsorbing phosphate onto GFO, and adding enough so that a detectable concentration of phosphate (0.1 to 1 ppm) is in equilibrium with the solids. Then remove the solid GFO and add it to seawater with no detectable phosphate. The now-detectable phosphate in the new seawater shows that the phosphate can be released from the GFO media when the aquarium’s phosphate concentration drops low enough.

One concern when using GFO is that it may add soluble iron to the system. This iron will likely benefit growing macroalgae, and I recommend adding soluble iron to systems that grow macroalgae. However, low bioavailability of iron may limit undesirable algae or cyanobacteria growth in some aquaria (and it can in parts of the ocean, as well), so adding iron might contribute to an algae or even cyanobacteria problem. In general, however, most aquarists find that the use of sufficient GFO causes a decline in algae, with the reduction in phosphate being more important to decreasing algae growth than the added iron is to promoting it.

A second concern with using GFO is that some aquarists find extensive precipitation of calcium carbonate near or on the GFO itself. It turns out that soluble iron can cause the precipitation of calcium carbonate. Such precipitation can turn bags of GFO into solid clumps, and may contribute to clogging pumps, but in general the effect, if noticed at all, is limited to objects very near the GFO. The extent of this effect may well depend on the degree to which calcium carbonate is supersaturated in the aquarium, as well as on the levels of magnesium and organics (both of which usually reduce the likelihood of calcium carbonate precipitation). Because of this potential for calcium carbonate precipitation, using this material in a reactor where it moves may be more important than for aluminum oxide media.

Finally, be sure to rinse these materials in fresh or saltwater before adding them to the aquarium, as fine particles may get loose in the aquarium, clouding and coloring the water, and possibly creating other problems. There is no efficiency drawback to this rinsing. Aquarists using the GFO in a fluidized bed reactor or canister filter can just run some fresh or salt water on it for a few minutes before putting it into the aquarium. A media bag of GFO can simply be rinsed with saltwater or RO/DI water a few times before adding it to the aquarium. Do not squeeze the GFO inside the bag when rinsing it, as that may break the particles into smaller bits that can escape the bag.

Soluble Metals to Bind Phosphate

There are several approaches that add soluble metals to bind and precipitate phosphate. The most popular involves adding lanthanum, which precipitates as lanthanum phosphate and/or lanthanum carbonate (which itself may contain some lanthanum phosphate). The lanthanum approach is widely used in the pool industry to reduce phosphate, and seems to often work well in aquaria. It is also very inexpensive, using products such as Seaklear (make sure it is a pure lanthanum version as mixtures with other metals also exist). Note that this method reduces alkalinity, as removing carbonate and phosphate as a lanthanum precipitate will reduce alkalinity.

One way to use it is to drip is slowly just upstream of a particulate filter to catch and remove a substantial amount of the precipitate that is formed. One drawback to the lanthanum approach is that much of the precipitated material may escape capture and simply settle out in the system somewhere. That may not be an issue, but many aquarists do not prefer to accumulate such material. A second concern is that some people have observed problematic reactions from aquarium inhabitants. While there are not a lot of such stories, it is enough for many aquarists to look for other options.

However, due to its low cost, this approach is especially well suited to outside of the tank operations, such as the removal of excess phosphate from phosphate-contaminated calcium carbonate rock that is later to be added to a reef aquarium.

Soluble iron has also been used in this way, but not nearly so often as lanthanum.

Summary of Phosphate Reduction Methods

My suggestion is for aquarists to target a phosphate concentration of 0.02 ppm phosphate, or less. Here is a list of ways that many aquarists export phosphorus and maintain appropriate phosphate levels.

1. One excellent method is macroalgae growth or an algal turf scrubber. Not only does it do a good job of reducing phosphate levels, but it reduces other nutrients, e.g., nitrogen compounds, as well. It is also inexpensive and may benefit the aquarium in other ways, such as being a haven for the growth of small life forms that help feed and diversify the aquarium, raising O₂, and reducing CO₂. It is also fun to observe the small creatures that grow in a refugium. I’d also include in this category the growth of any organism that you routinely harvest, whether corals (Xenia sp.) or other photosynthetic organisms.

2. Skimming is another good method, in my opinion. Not only does it export organic forms of phosphate, reducing the potential for them to break down into inorganic phosphate, but it reduces other nutrients and increases gas exchange. Gas exchange is an issue that many aquarists don’t ordinarily recognize, but it is a primary driver of reef aquarium pH problems.

3. The use of limewater, and possibly other high pH alkalinity supplements, is also a good choice, although the amount exported in this way may be fairly low. It can be very inexpensive and it solves two other big issues for reefkeepers: maintaining calcium and alkalinity. Simply keeping the pH high in a reef aquarium (8.4) may help prevent phosphate that binds to rock and sand from re-entering the water column. Allowing the pH to drop into the 7s, especially if it drops low enough to dissolve some of the aragonite, may serve to deliver phosphate to the water column. In such systems (typically those with carbon dioxide reactors), raising the pH may help.

4. Commercial phosphate binding agents clearly are effective, and are used by many aquarists. They can be expensive and may have other drawbacks, but can drive inorganic phosphate to very low levels, if that is a goal.

5. Driving bacterial growth is another option. Not only does it do a good job of reducing phosphate levels, it reduces other nutrients, such as nitrogen compounds, as well. It is also fairly inexpensive and may benefit the aquarium in other ways, such as providing a food source for certain organisms. Its drawbacks are that it makes it difficult not to drive the nutrient levels too low, and the fact that it consumes oxygen as the bacteria use the added organics as a carbon source. I use vinegar, but vodka or commercial additives are also a good choice.

In my own tank I use macroalgae growth (mostly Caulerpa racemosa), GFO in a reactor, skimming, vinegar dosing, and GAC. Together, these keep my system at a reasonable level of phosphate, but by no means a ULNS (ultra low nutrient) system.

Summary

Problems involving phosphorus and subsequent algae growth can be among the most difficult to solve in a reef aquarium, especially if the live rock and sand have been exposed to very high phosphate levels, after which they may be acting as a phosphate reservoir. Fortunately, steps can be taken even in the absence of any algae problem that will benefit reef aquaria in a variety of ways, not the least of which is reduction of phosphate levels. These include skimming and growing macroalgae. All reefkeepers, and especially those designing new systems, should have a clear idea in mind about how they expect phosphorus to be exported from their system. If allowed to find its own way out, it more than likely will result in undesirable microalgae that many reefkeepers are constantly battling

If you have any questions about this article, please visit my forum at Reef2Reef.

References:

1. Chemical Oceanography, Second Edition. Millero, Frank J.; Editor. (1996), 496 pp.

2. Kinetics of precipitation of calcium carbonate in seawater: role of phosphates and hydrodynamics of the environment. Pokrovsky, O. S.; Savenko, V. S. Mosk. Gos. Univ., Moscow, Russia. Okeanologiya (Moscow) (1993), 33(5), 681-6.

3. Dissolution kinetics of calcium carbonate in sea water. V. Effects of natural inhibitors and the position of the chemical lysocline. Morse, John W. Dep. Oceanogr., Florida State Univ., Tallahassee, Fla., USA. Amer. J. Sci. (1974), 274(6), 638-47.

4. Calcium carbonate retention in supersaturated sea water. Pytkowicz, R. M. Sch. Oceanogr., Oregon State Univ., Corvallis, Oreg., USA. Amer. J. Sci. (1973), 273(6), 515-22.

5. Measurement of alizarin deposited by coral. Lamberts, Austin E. Dep. Zool., Univ. Hawaii, Honolulu, Hawaii, USA. Editor(s): Cameron, A. M.; Campbell, B. M.; Cribb, A. B. Proc. Int. Symp. Coral Reefs, 2nd (1974), Meeting Date 1973, 2 241-4.

6. Effects of elevated nitrogen and phosphorus on coral reef growth. Kinsey, Donald W.; Davies, Peter J. Limnol. Oceanogr. (1979), 24(5), 935-40.

7. Photosynthesis and calcification at cellular, organismal and community levels in coral reefs: a review on interactions and control by carbonate chemistry. Gattuso, Jean-Pierre; Allemand, Denis; Frankignoulle, Michel. Observatoire Oceanologique, LOBEPM, UPRESA 7076 CNRS-UPMC, Villefranche-sur-mer, Fr. Am. Zool. (1999), 39(1), 160-183.

8. ENCORE: the effect of nutrient enrichment on coral reefs. Synthesis of results and conclusions. Dennison, W.; Erdmann, M.; Harrison, P.; Hoegh-Guldberg, O.; Hutchings, P.; Jones, G. B.; Larkum, A. W. D.; O’Neil, J.; Steven, A.; Tentori, E.; Ward, S.; Williamson, J.; Yellowlees, D. Marine Pollution Bulletin (2001), 42(2), 91-120.

9. The effects of HEBP, an inhibitor of mineral deposition, upon photosynthesis and calcification in the scleractinian coral, Stylophora pistillata. Yamashiro, Hideyuki. J. Exp. Mar. Biol. Ecol. (1995), Volume Date 1995, 191(1), 57-63.