Phosphate is an essential component in fish nutrition, but is also produced as a waste product by fish. Phosphate is practically non-toxic to fish, but excess phosphate can cause unwanted algae and will inhibit coral growth.

Maintaining a low level of phosphate is essential for reducing unwanted algae and maintaining coral growth rates. Phosphate tends to accumulate as an aquarium ages, so regular monitoring of phosphate is required to determine the health of the aquarium.

Phosphate in natural seawater generally varies from 0.01 mg/l to 0.06 mg/l (in terms of phosphorus) in reef areas. However, in a reef aquarium, aim for 0.03 mg/l to maintain good coral growth and keep nuisance algae at to a minimum.