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Phosphates (PO4) can be found in every aquarium, even if in small quantities. Phosphates are one of the main causes of algae in an aquarium. The presence of the materialized form of any type of phosphate is known as ortho-phosphates. Ortho-phosphates are created by the breakdown of simple and complex organic phosphates and are major contributors to the growth of algae. All living things 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.


Phosphates are produced as wastes are broken down in the aquarium. They can also be introduced from external sources. Everything from food, to the chemicals used to buffer the water.

Phosphate sources include:

•           uneaten food

•           plant decay

•           dying algae

•           fish feces

•           dead fish

•           aquarium salts

•           pH buffers

•           kH buffers


The largest source of phosphate is pH buffers.

(Those buffers labeled as pH 7.0, pH 6.5 or kH+ etc)

These buffers contain huge amounts of sodium bi-phosphate.

Just to diverge a little it is probably appropriate to explain why this is.

Basically and simplistically there are two commonly used pH buffers.

pH Up    ---- sodium bi-carbonate

ph Down---- sodium bi-phosphate

As an example: If a buffer were to contain equal amounts of sodium bi-carbonate and sodium bi-phosphate it would force (for the purpose of this example) the ph to neutral (7.0)

If the proportions are varied it is possible to force the pH to any desired level.

Using these buffers is just asking for an algae problem.

In an aquarium that does not contain plants it probably does not matter as an algae control chemical can be used.

However this goes against our philosophy of not adding ANY chemical or additive to any aquarium without good cause.

These buffers are simply the lazy way of achieving pH balance.

A phosphate free kH generator can be used to prevent pH crash and acid based pH downs can be used to lower pH.


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.


Phosphates are present in both organic and inorganic forms. Test kits are only able to test for inorganic phosphate, so keep in mind that you are only testing a portion of the total phosphate in your aquarium.

When test results show levels of 1.0 ppm (or 1.0 mg/L), the conditions become favorable for algae growth to start. At 2 to 3 ppm, algae overgrowth is likely to occur. Ideal phosphate levels are 0.05 ppm, or less.

Stopping phosphates from getting into your aquarium or from forming within the aquarium is the best way to avoid the harmful consequences of accumulating phosphates.

Phosphate cannot 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.


Phosphates do not directly harm your fish, even at high levels. However, the algae blooms that result from elevated phosphates can ultimately cause problems for the aquarium inhabitants. For instance, green water can deplete the oxygen, which in turn can harm the fish.


The best way to reduce phosphate in your aquarium is to never let it get high in the first place. However, if your phosphates are high, you can reduce it by taking the following steps.


Large water changes will help bring phosphates down quickly, but if the underlying sources are still there, it will only be temporary. Until all causes are cured, continue to perform frequent large water changes to keep phosphate levels manageable.


Scrape the inside of the glass, remove the rocks and other decorations and scrub them well. Let everything settle a bit, then give the substrate a good vacuuming. Wait a few days to give things a chance to stabilize, and then clean the filter.


Phosphate absorbing media is very effective. It can be added to virtually any filter.

NOTE: Generally using chemicals should be your last resort.


Once you bring the phosphate level down, make sure it stays low. Here are some ways to avoid soaring phosphate levels.


The number one source of phosphate in the aquarium is flake food. Cut back on the frequency and amount of food. Just a pinch once a day is sufficient for most adult fish. Remove any uneaten food promptly.


Phosphate is used as a preservative in flake foods. All brands are not created equal, so do your research and choose those brands that have lower phosphate levels.


Test your water source. It is not unusual for tap water to contain 1 ppm of phosphate. If the level is high, seek an alternate source for your aquarium water.


Frequent water changes will help keep phosphate levels from rising. Change ten to fifteen percent weekly, using a low phosphate water source.


Keeping the tank free of debris will help avoid phosphate buildup. Vacuum the bottom frequently to remove uneaten food, plant decay, and fish waste.


Buffers that condition the water, alter or stabilize the pH, add trace elements, or change the hardness, often contain phosphates. If they aren’t absolutely needed, don’t use them. When you do use them, research the product and choose the one that contains the least amount of phosphate.