Phosphate (PO4) is a chemical compound and contains the element phosphor.
An overconcentration of phosphate in salt water aquariums leads to an over fertilisation of the corals that live in symbiosis with the zooxanthellae, which in turn leads to brown coloured corals. The occurrence of over concentrated phosphate in water can result from insufficient skimming, sporadic exchange of water or from feeding (especially in the case of frosted food). Filamentous algae are often a visible sign of phosphate overconcentration.
A regular measurement of the phosphate content is thus very important in order to prevent the overconcentration of phosphate, in order to trigger counteractive measures at the right time (e.g. by using phosphate adsorbents, water exchange, etc.). The optimal phosphate content in salt water for should range between 0.03 and 0.1 mg/l for stone corals (SPS) and between 0.1 and 0.2 mg/l for LPS corals. Other coral species, such as soft corals tolerate values between 0.1 and 0.4 mg/l.
We would like to add that an aquarium should in fact contain low concentrations of phosphate (below the above listed values), since the corals metabolize it as a nutrient and since it therefore plays a major role for a healthy reef aquarium.