All living matter, both animal and plant, contains about 1% Phosphorus in the form of Phosphate (PO₄^3-). Every time the fish are fed, some Phosphate is introduced into the aquarium. Waste products of the fish and bacterial breakdown of food remains also introduce Phosphate into the water.

In nature a true Phosphorus cycle exists. Phosphate is an important plant fertilizer that stimulates plant growth at very low concentrations. The Phosphate level in unpolluted natural waters is always very low, and Plants use so much that there is always a demand for it.

In the aquarium, the fish density is much higher than in nature. In most aquariums the plant growth is insufficient to use up all the Phosphate that is added with feeding, therefore it accumulates in the water. This leads to several undesirable effects, in particular blue, green and hair algae find favorable conditions and grow very fast. Marine invertebrates like corals may loose their zooxanthellae, (tiny brown algae that live in the tissue of invertebrates leading to degeneration and eventually to the death of these sensitive animals.

Especially in a reef aquarium, the Phosphate level should be checked every week. The concentration should be as low as possible with 0.3 ppm as an absolute maximum for the reef aquarium. Higher concentrations than this value are not in any way toxic for marine or freshwater fish - the danger lies only in the fact that the water gets over fertilized, which results in algae growth and harm to invertebrates.

Recommendations

To prevent build up of Phosphate

Keep the fish density low.

Do not overfeed.

Stimulate plant growth (in the marine aquarium, higher algae like Caulerpas, as well as soft corals will efficiently use Phosphate).

Remove detritus, dead plants and other dirt.

Perform regular water changes. Use Red Sea’s Red Sea Salt for the marine aquarium. This top quality salt contains neither Phosphate nor Nitrate.

For the marine aquarium: use good protein skimmers like Red Seas Berlin or Prizm Protein Skimmers. These highly efficient skimmers clear up waste material, before it can decompose to Phosphate and Nitrate. This Phosphate and Nitrate build up is limited or prevented.