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What is it?[edit]

Phosphate (PO4) is an inorganic chemical and is a salt of phosphoric acid.

Its role in aquariums is that it contains phosphorus, an essential nutrient for plants (that includes algae) and as a useful buffering agent within animal cells.

It is an essential chemical for the growth of roots and flowering in plants as well as a growth chemical for all life.


Phosphorus (P) is never found as a free element in nature as it is usually combined with other elements like oxygen into phosphate as far as the aquarium hobby is concerned.

How it gets into the tank[edit]

It is usually added via fish food and is an unwanted chemical in a non-planted tank as it promotes growth of that unwanted plant - algae. However in a heavily planted tank phosphorus may be in short supply and Potassium phosphate is added to feed plants.

  • It is also sometimes present in tap water as it's harmless in drinking water to humans.
  • Some cheap forms of activated carbon may leech phosphate into the tank water over time.

Not so evil[edit]

Since the 1970s phosphorus has been seen as the evil chemical that causes the growth of unwanted algae and therefore there are many products on the market which are targeted at absorbing or removing this chemical.

However in the 1990s aquarists realised that this isn't the whole story when planted aquariums became more popular. Plants need this chemical to grow and studies by people like Tom Barr and Diana Walstad have discovered that ammonia and other nitrogen forms like nitrite and nitrate in the water cause far more algae growth when compared to phosphate.

Yes, in a non-planted tank have a phosphate remover to remove algae. But algae only grows if nitrogen is also present so perhaps your filter is not working very well and removing ammonia/nitrite/nitrate should be a higher priority?

  • In salt water tanks the need to remove phosphate to a low level is very important to ensure the health of corals and to prevent unwanted algae.

Testing for the presence of phosphate[edit]

There are commercial test kits to allow you to test the levels of phosphate in your tank water.

  • Some countries supply tap water with levels of phosphate. Whilst this is safe for humans to drink and wash in, it may cause algae problems in uncycled or inadequately filtered tanks. Get a phosphate test kit from your aquarium store.

Removing phosphate from the water[edit]

  • Pass water through a resin so that the phosphate is absorbed and trapped. When the resin is full it can be recharged or thrown away. These can only be used in freshwater.
  • Add a chemical to the water so that phosphate is transformed into a harmless compound that plants can't absorb.
  • If your tap water is free from phosphate, do water changes to dilute it.
  • Aquatic plants will remove it as they grow.