Nitrate is considered the non-toxic end product of the ammonia cycle. While not directly a problem for fish at low levels, excess nitrates can cause stress and other water quality issues. Nitrate is continually produced as fish respire and released waste products move through the ammonia cycle, and can accumulate rapidly if not properly maintained. The nitrate level should be monitored regularly and routine maintenance is needed to keep concentrations low for a healthy aquarium. High levels of nitrate can also contribute to undesirable algae growth.

The nitrate level in an established tank will always be above 0 ppm. 
Acceptable level of nitrate in most freshwater aquariums: 20-40 ppm nitrate
Acceptable level of nitrate in most saltwater aquariums: below 20 ppm nitrate