Cause & effect of aquarium phosphates

Triggerfish Crossbow-clown, Balistoides Conspicillum)
(Last Updated On: March 13, 2017)

All living organism contain phosphorus. Phosphorus is an important element of life as a component for cell membranes, as an energy source, and for other bio-chemical processes.

Phosphorus is a very reactive component making it readily absorbed and generally available in aquatic environments as either an organic or inorganic phosphate.

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Phosphates (PO4) can be created within the closed aquatic system or imported from the outside.

Phosphate as a by-product of mineralization of dead matter such as plants, bacteria, feces, uneaten food, fish slime etc. are all internal contributors.

Dead plant material or rotting food particles settle either on the substrate or within the filter. Rinsing filter materials and vacuuming the gravel at every water change can significantly reduce potential phosphate accumulation from these internal sources.

Replacement water can also contain phosphate, sometimes surprisingly high concentrations, even if RO units are in use.

Additives such as pH stabilizers or carbon, and frozen fish food are potential external phosphate sources. Avoiding phosphate containing products as well as testing of the replacement water for phosphates can further help prevent accumulation. If in doubt, additives, carbon, pH buffers, and the water should be tested and replaced if necessary.

Prohibiting phosphates from entering the water or from forming within the aquarium is the best safeguard from the harmful consequences of accumulating phosphates.

Inorganic phosphate or orthophosphate is the soluble form. It is readily available and quickly absorbed by plants. Organic phosphate refers to phosphate that is part of a cell structure or organically bound in other ways. Organic phosphate must be broken down by bacteria in order to become soluble orthophosphate.

The biggest source of organic phosphate is fish food. 5 grams of flake food can increase the organic phosphate level by 0.4 ppm. The filters and substrate have to be cleaned regularly before the organic phosphate is mineralized to inorganic orthophosphate.

Some marine and especially reef aquarium set-ups rely on less frequent water changes. The reason for one is a delicately balanced filtration based on live rock and/or the need for nutrient supplementation for coral growth, among others. To compensate for less frequent water changes a protein skimmer is attached, which will remove many waste particles that would otherwise be broken down to soluble orthophosphates.

Unfortunately, protein skimmers do not work in freshwater aquariums and can not be substituted for less frequent water changes. More than 90% of the phosphate contained in the aquarium is organic phosphate. The common test kit measures the inorganic soluble orthophosphate, not the organic form or the total phosphate content.

Generally the measurable phosphate level should be below 0.05 ppm.

Planted tanks have the advantage that plants are capable of storing and consuming phosphates. Plants can only take up in-organic orthophosphate, thus reducing the levels. Saltwater tanks can imitate that by planting macro algae into a refugium or sump.

In reef aquariums Kalkwasser can just about eliminate phosphate. At a pH above 8.9 phosphate precipitates in the water as insoluble phosphate and flocks out. Marine aquariums kept above a pH of 8.4 allow some phosphate to be bound to rocks and substrate in an insoluble form. Nevertheless it will become soluble if the pH drops below 8.

In closing, phosphate can not be entirely removed from the aquarium since organic phosphate is constantly converted into in-organic soluble orthophosphate. Nevertheless, phosphates can be controlled with a good maintenance schedule aimed at keeping organic phosphates at a minimum.

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9 thoughts on “Cause & effect of aquarium phosphates

  1. Patricia Huston says:

    We have a 150 gallon fish tank. I have been checking it every week & and when I checked it again the phosphate it was even bluer then the color cart I’m talking off the wall high. We took all the fish out, took all the gravel out and cleaned the algae off the glass and drained the water out. I washed all the gravel and put it back in and filled it back up but the phosphate is the same off the wall high. I just don’t know what to do at this point. Oh I have well water and the only thing is I have a water softner on our house & checked the water coming out of the hose and it is normal. Is there anything else that I can do to bring it down. Thank You.

    • Thilo
      Thilo says:

      In situations where good maintenance cannot prevent high phosphate levels, we need to resort to phosphate removers. Basic versions are based on either aluminum oxide, iron oxide, or ferric oxide. As always with test kits, they are highly sensitive so make sure they are unaltered and not expired.

  2. Ron Margiotta says:

    Recently have algae on glass of tank–125 Gal. Peacock Cichlids–use cannister filters 1-Fluval FX6 and 1 fluval 406
    All Silk plants

    Phosphate levels high. Just yesterday used additive Phos Buster.Is this a good method to reduce phosphate levels?
    Appreciate any help.

    • Thilo
      Thilo says:

      Phosphates should be kept low. The most single source of phosphates is food. Check your water supply as well. Knowing where your phosphates come from will greatly help to avoid them in the future. Please check for silicates in case your algae tends to be more brown than green.

    • Scott @ Algone
      Scott @ Algone says:

      Algone actually helps lower nutrient spikes during the cycling of the aquarium. While spikes in ammonia, nitrite, and eventually nitrate are unavoidable during the cycle, keeping them as low as possible makes the process more gentle.

  3. Vincent says:

    I have a 45 gal. Tank with a phosphate level of 10 . The tank is crystal clear and I started using a phosphate absorber from api . It’s been only 24 hrs. Will the levels eventually diminish because so far no change. I have discus in the tank and so far they seem to be fine. The ph is at 6.8 . Your response would be appreciated.

    • Thilo @ Algone
      Thilo @ Algone says:

      The listed ingredient of activated alumina indicates a removal by ion exchange. The effectiveness and absorption rate is influenced by pH and kH levels. You might simply replace the pouch to see results as the pouch might have been quickly exhausted. If not, call the manufacturer to discuss options.

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