Nitrate is the result of the bacterial breakdown of ammonia > nitrite > nitrate which is the final stage of the 
natural biological metabolic waste conversion also known as the nitrogen cycle.
Although less toxic than ammonia/ammonium and nitrite, nitrate as a nitrogen compound also causes stress at all levels making a fish’s organs work harder to adjust to it’s new environment. 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.
The process of breaking down ammonia > nitrites > nitrates is known as the nitrification process. It takes place in an aerobic environment. Nitrifying bacteria settle on gravel and build colonies. They need nutrients (ammonia and nitrite) and oxygen in order to perform their tasks. 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. The higher the nitrate concentration the more stress for the fish. Extremely severe stress is reached at levels exceeding 60 ppm. Most of the plants fail before reaching this level. This due to an accumulation of live forms feeding from the waste, and the consequently higher biomass (animals living in the aquarium) leads to an increasing demand of oxygen.
Therefore nitrate levels (NO3) should be kept under 10 ppm. As most of the test kits measure nitrogen – nitrate (N-NO3) the reading has to be multiplied by 4.4 to get the accurate nitrate (NO3) concentration.
Nitrate is also a key nutrient source for algae. Most of the pesky and unwanted algae thrive on poor water quality, high nutrient levels and excessive nitrate. Many initially cycling tanks experience an algae bloom due to this effect.
