Balancing the nitrogen cycle is vital for an aquarium.
(1) adding food and nutrients,
(2) production of urea and ammonia by fish,
(3) Conversion of ammonia into nitrites by bacteria of the genus Nitrosomonas,
(4) nitrites are converted into nitrates by bacteria of the genus Nitrospira. Nitrates, less toxic than nitrites and ammonia, are consumed by plants. The periodic renewal of the water removes excess nitrates.
(5) Evaporation, (6) light, (7) oxygen cycle, (8) O2 produced by plants, (9) CO2 produced by fish.