Our water has 2ppm of PO4, and about 20-30 ppm nitrate. The increasing stress results in the loss of ability to fight diseases, the ability to heal itself, and the ability to reproduce. It is essential for you, the aquarium hobbyist, to maintain a proper environment for your aquatic companions. High nitrate levels are a sign of poorly maintained aquariums and will cause problems in the long run. It takes place in an aerobic environment.
Nitrifying bacteria settle on gravel and build colonies.
The result is nitrate. The removal of nitrate, if not utilized by plants, takes place in an anaerobic environment and is called denitrification. Nitrates are potentially dangerous due to the effects on the water chemistry and on a healthy environment for your fish while nitrates are accumulating. The higher the nitrate levels the higher and severe the consequences due to the stress on your fish and the favorable conditions for a serious algae outbreak. Nature provides an almost nitrate free environment with levels around 5 ppm or less.