Any salt or ester of nitric acid, and the end product of the portion of the nitrogen cycle taking place in most closed aquarium systems (the conversion of nitrates to nitrogen gas in natural systems is carried out by anaerobic microorganisms not normally present in sufficient numbers in aquaria, though this process can be induced by installation of a nitrate reactor). While nitrates are less toxic than ammonia or nitrites, they are still potentially harmful to fishes and other aquatic organisms if allowed to accumulate unchecked, and must therefore be diluted via regular water changes. Aquatic plants also help since they are able to metabolise nitrates to an extent.