A component of certain enzymes and proteins, ATP, ADP, and DNA and phytin. In the aquarium it exists in 4 different forms:
Dissolved inorganic phosphorus. Available to algae & plants.
Particulate inorganic phosphorus. Mostly not available to plants & algae.
Dissolved organic phosphorus. Available to bacteria & algae.
Particulate organic phosphorus. Not available to plants & algae.