Cause & Effect of Aquarium Phosphates

Triggerfish Crossbow-clown, Balistoides Conspicillum)

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.

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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Benjamin D Ice

I have a 75 gallon Oscar tank. The tank water totally green. I don’t feed them very much and despite 90% water changes weekly, nothing is removing the green. I tried top fin algae remover, but that isn’t really doing very much. At this point, I’m not sure what to do. I have city water so I don’t think that’s the issue and a few more things where the phosphate levels are totally normal. The tank is decorated with big rocks and sand, I don’t know if that could be an issue or not.


By your comment I take that you have more than one Oscar, for which a 75 Gallon aquarium is too small for even just one. In essence, the waste created within the aquarium outperforms the tanks ability to handle it. The excess waste then fuels green water which is nothing less than algae thriving and growing at a rate that turns the water green. See this link for more:


I have a small fresh water tank mainly planted with Micranthemum Montecarlo and a growing community of shrimps. I am currently dosing a constant flow of diluted tropica premium enriched with extra phosphates. The reason for this strategy is due to the fact that, even though The tropica mix itself has a NPK in the right proportion, Testing indicates that adding a given amount of Po4 a day (even if I add extra po4 as to complement the comercial product and I feed the shrimps) results in zero to none mg/L of phosphate in small hours. Where’s all that phosphate… Read more »


Make sure the test kit is accurate, most of them are vague. Is there a phosphate deficiency in your plants? This will cause the phosphates to be absorbed more quickly. Double the output for a brief period, this should give you an indication.


I have a planted tank with no fish atm and while I’m using a soultion made up of tank water and broken down fish food to help with cycling the tank, I worry that it isn’t enough to have healthy plants – so I’m wondering if a drop or two of babybio will help. Ingredients are nitrogen, phorphorus pentoxide and potassium oxide.


Pants can store nutrients and the fish food will provide enough nutrients for their growth. You can check this link to see if you have deficiencies that need to be addressed.

Patricia Huston

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… Read more »


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.

Ron Margiotta

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.


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.


Did using algone during cycling will affect a process?

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.


0.73 is good for reef aquarium


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

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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