What’s phosphate?

When talking about phosphate in aquariums, usually so called orthophosphates are referred to. These are (mostly) negatively charged ions containing phosphorus, oxygen and at least some of them hydrogen. Other inorganic and organic forms of phosphates are ultimately turned into orthophosphates by biochemical activities. These orthophosphates are what is measured by most of the various phosphate testing kits available in the stores. Phosphorus is contained in practically all living organisms, but also in a number of common minerals like apatite (Calcium phosphate). Phosphates can potentially be released from all these phosphorus sources.

Desirable concentrations

Phosphates have two major influences on marine aquariums: Algae growth (especially of the ones we don’t like looking at) can be promoted by high levels of phosphates, and corals don’t react very well to high phosphate levels (stunted growth, general decay).
The production of the calcareous skeleton of stony corals is significantly inhibited by high phosphate levels, but most of the more sensitive soft corals don’t react happily to high phosphate levels as well. Therefore, such high concentration should be avoided. But how high is high?

The water of natural coral reefs contains very little phosphate, typically around 0.005 ppm, significantly lower levels can result in a kind of coral bleaching (Tissue cannot regenerate anymore), so this should be smallest desirable concentration.
Serious decrease of coral growth was detected at concentrations above 0.1 ppm. This can be considered ideally as the maximum level you should have in your reef tank with stony corals. 0.25 should do if you only have easy coral species, below 0.1 ppm seems mandatory for the more sensitive ones.

While corals can react negatively to an increase of phosphates, fish apparently are not affected. Whenever the fish are doing great, and the corals are declining, it’s time to check the phosphate level of a tank.

Input: Common sources of phosphates

Saltwater
Both salt and water can be possible phosphate sources that should be avoided simply by using commercial salt brands that don’t contain phosphate, and by using RO or deionised water. Tap water may contain amounts of phosphates fine for human consumption, but unhealthy for corals. If you are unsure about the phosphate content of your water and salt, you can use standard phosphate testing kits to check it prior to and after mixing your saltwater.

Rocks and substrate
Dead reef rock (also referred to as base rock), tufa rock, but also crushed coral substrate may contain serious amounts of phosphates. Some is bound very tight (pretty dissoluble minerals), some may be dissolved given the proper biochemical environment. If you want to know approximately how much soluble phosphate is contained in a specific rock or substrate, it is possible to test it in a bucket with heated and moved salt water for some days. Contents of 1 mg phosphate per kilogram of rock or crushed coral, and higher have been determined that way. A tank in which such rock is used will have high phosphate concentrations for a long time. If phosphate rich rock material is already in your system, there are two possible solutions: Take out the rocks, or increase the phosphate output of your system (see below).

The use of dead reef rock may be seen as one of the major sources of long term high phosphates in marine aquariums. If they are used they should be rinsed very well. Dead reef rock should also be scrubbed. Proper bleaching may be necessary to remove some of the more soluble phosphate containing agglomerations and minerals. Fresh and cured life rock, however, has a much lower phosphate content and should be preferred when setting up a new tank.

Frozen foods and thawing water
All foods used for fish and invertebrates contain phosphate (roughly 0.2-3% of its dry weight), but since you have to feed your animals, this input cannot be decreased at first glance. What you can do is keep the thawing water (that is not eaten anyway) out of the aquarium. Why all that rinsing work?

The thawing water of various frozen foods has been analysed with regard to its phosphate concentration (e.g. by Schwirtz). Contents between 0.74 mg/l (fish eggs thawed for 20 minutes) and 15.53 mg/l (mussel flesh thawed for 2 hours) were measured in 100 ml of RO water, in which about 3 grams of frozen food were thawed. Accordingly, in a 100 gallon tank fed with one un-rinsed 3 gram cube of mussel flesh the phosphate concentration is being raised by 0.0039 mg/l per feeding only by the thawing water. In 100 days without considering large water changes, this would correspond to a phosphate concentration of 0.39 mg/l, a concentration not healthy for corals, and the phosphates introduced by the actual mussel flesh are not even considered in this example. So, rinse your frozen foods!

Activated carbon
Steven Pro found that several samples of activated carbon intended for aquarium use contained noticeable amounts of soluble phosphates. He used 50 ml of carbon and 250 ml of deionised water for his tests. The phosphate concentrations detected may seem small considering the carbon volume and water volume of a standard reef tank, but the frequent or permanent use of activated carbon may be an additional phosphate source that has to be considered, especially in the lower range of the phosphate level desirable for a reef tank with difficult coral species.

Output: Removal of phosphates

Water changes
Phosphates, just as any other substances not existing in freshly mixed saltwater, are removed by water changes. Changing 10% of water will decrease the phosphate concentration ideally by 10%. However, if phosphate reservoirs should exist in your tank, and your phosphate level was in balance with these reservoirs prior to the water change, the phosphate concentration may rise again in hours or days by formerly precipitated phosphates dissolving again. Someday these reservoirs will be exhausted (if output of phosphates is larger than input), but depending on the system, this may take a long time and lots of water changes.

Precipitation
An old, but still common technique of removing phosphates from the water column is precipitating them as hard to dissolve minerals. Aside growth of bacteria and plants (see below), this still is the most important method used in large scale water cleaning systems like sewage plants. It’s also the reason why the phosphate levels in the sea are very low: Phosphates are precipitated especially in the vicinity of river mouths where lots of iron containing sediments are transported into the ocean, and the resulting minerals are buried in the sediments on the bottom of the coastal waters.

Precipitation of phosphates in aquariums has one obvious danger: the creation of new phosphate reservoirs. But how can such apparently insoluble substances be dissolved again? Well, just the same way they got precipitated: small and large scale changes in water chemistry (e.g. lowering pH).

Imagine Calcium being used up by extensive coral growth: minerals consisting of Calcium und phosphate (a discussed phosphate reservoir) will be dissolved until the balance is restored. While Calcium is used up again by further coral growth, the phosphates will mostly stay dissolved in the water.

Next, imagine a partial water change: You’ll remove Calcium and phosphate, which are in balance with a phosphate reservoir. The fresh water only has Calcium, so again, phosphates can be dissolved and the phosphate concentration will rise.
As a result, personally I prefer to remove phosphates once and for all from tanks instead of precipitating and accumulating more and more phosphate sediment. While these phosphate sediments are removed from sewage plants with excavators and are possibly valuable to produce fertilizer, this is difficult and messy in the aquarium, and can only partially be accomplished by gravel vacuuming. To me precipitation is not a long term solution.

    

Growth of macroalgae and animals
Every growing life will use up little amounts of phosphates. With each fireworm, macroalgae, coral fragment etc. you take out of your tank, you’ll remove some phosphate that was taken up during the growth of the life form. Vice verse, everything that dies or is eaten will increase the phosphate level of your tank.

Macroalgae, the most common life forms used to intentionally remove phosphates (and nitrates) have been subject to studies examining their phosphate (and nitrate) content. Caulerpa recemosa was found to contain 0.08% phosphorus (dry weight). That’s about 0.24% (ortho)phosphate. 1 gram of dried algae contains 2.4 mg phosphate. However, since wet algae contain mostly water (up to 98%) you’d have to harvest at least 10 grams to remove 2.4 mg phosphates from your tank. In a 100 gallon tank this is a phosphate level drop of about 0.006 ppm (this amount of algae removal corresponds to a decrease of the nitrate concentration by 0.06 ppm).

Removing phosphates by harvesting macroalgae or fast growing corals is perfect for tanks with a relatively small input of phosphates, but this method alone is usually not efficient enough to remove phosphates from extensive feeding or polluted dead reef rock.

Growth of bacteria
Just like macroscopic life forms bacteria take up phosphates while growing. But how do you remove their biomass from your system? There are basically two possibilities: regularly rinsing biological filters like bio-balls, zeolites and sponges, and skimming, which can remove entire bacteria or parts of dead bacteria into a cup.
Similar to growth of higher life, bacteria growth can help you to keep low phosphate concentrations down and stable, but it is mostly not efficient enough to deal with high concentrations or phosphate reservoirs.

Skimming
My own analysis of processed skimmate of a heavily fed tank revealed nitrate concentrations of more than 200 ppm, which was about 20times the concentration of the tank water. Phosphates, however, only showed twice the concentration of the tank water, 1 ppm. So, this specific skimmer in this specific setup was much more effective in removing nitrates (or to be more exact: organic substances that were turned into nitrates by bacteria) than it was in removing phosphates. Results of other tanks may vary (the ones I tried so far were similar), but can easily be checked by measuring nitrates and phosphates in skimmate. The skimmate should be kept warm (25°C) and be aerated for about a night or longer to give the bacteria a chance to turn proteins in the skimmate into measurable substances (nitrates, phosphates).

Skimming can potentially help you to remove phosphates, but if you want to know how much help it exactly provides, you have to analyse the skimmate. For high phosphate values it is unlikely to be a very big help (removed organics typically do not consist of very much phosphate), at least I have yet to measure really high phosphate contents in skimmate.

Adsorbing media

The industry provides a number of phosphate adsorbing media (e.g., PhosGuard, Phosphate Remover, Phosphate Sponge, RowaPhos, X-Phosphate,…), and the ones I tried worked as promised if applied as recommended by the companies. The adsorbing media consist of aluminium oxides and iron hydroxides specifically manufactured to have large surfaces. Usually, the media is applied in a fluidised bed reactor or simply in a very fine net at spot in the tank with medium current. On the surfaces of the granules phosphates are bound (adsorbed). When the surfaces are “full” and the concentration starts rising again, the media and the bound phosphates are taken out of the aquarium. Using this media is very helpful with any elevated phosphate level. The only danger with this technique is to drop the phosphate concentration below the level needed by corals for tissue regeneration. Therefore, it is important to monitor the phosphate level during the use of adsorbing media. It is also important to rinse the adsorbing media (preferably with RO water) at least every few days to remove the thin layer of bacteria that builds up around the grains and makes the media quickly ineffective by sealing the surfaces.
Because organic molecules and some metals may potentially also be bound to phosphate adsorbing media, and thus decrease its efficiency, it can be recommended to use these media preferably after a few days of filtration with activated carbon.

Since I work in the environmental field (e.g. projects about ground water pollution), I’ve also tried iron hydroxides intended for large scale phosphate removal from drinking water in aquariums and found them to be just as effective as the aquarium products. However, I can only recommend them to people, knowing exactly how to apply the products for groundwater and drinking water treatment.
Adsorbing media are a good solution for high phosphates in tanks with phosphate reservoirs from dead reef rock without having to remove the rock from the aquarium. They are also especially helpful in tanks with low to medium nitrates, but high phosphates, a situation that often occurs in tanks with anaerobic, de-nitrifying filters like DSBs and large fish. A definite minus is the high price these media command.

Control

Maintaining a healthy level of phosphates can be essential in a marine aquarium. The phosphates in your tank are basically input minus output. If you need to decrease the concentration, increase output and decrease input. But, whatever you do, do it slow and aim for a long term balance. Too fast drops of phosphates or drops below the level found in natural seawater may instantly and permanently harm your corals.

References

Anja Schwirtz: Bestimmung der Phosphatmenge im Abtauwasser von diversen Frostfuttersorten; http://www.aquacare.de/info/forsch/dla_phos.htm (Determination of phosphate levels in the thawing water of various frozen foods)
S. T. Larned: Nitrogen- versus phosphorus-limited growth and sources of nutrients for coral reef macroalgae.- Marine Biology (1998) 132: 409-421. http://www.botany.hawaii.edu/Bot482/Kaneohe%20Bay%20algae%20N-P%20Larned%20Mar%20Biol.pdf
Randy Holmes-Farley: Iron Oxide Hydroxide (GFO) Phosphate Binders.- Reefkeeping 2004/11. http://reefkeeping.com/issues/2004-11/rhf/index.php
Randy Holmes-Farley: Chemistry and the aquarium – Aluminium in the reef aquarium.- Advanced Aquarist 2003, July. http://www.advancedaquarist.com/issues/july2003/chem.htm
Steven Pro: Activated carbon and phosphates. Conscientious aquarist vol. 4
http://www.wetwebmedia.com/ca/volume_4/V4I2/carbon/carbon_comparisons.htm