Nitrogen Cycle in aquariums. Legend: (1) Addition of food and nutrients, (2) Production of Urea and Ammonia by Fish, (3) Ammonia is converted to Nitrites by beneficial Nitrosomonas bacteria, (4) Nitrites are converted to Nitrates by beneficial Nitrospira bacteria. Less toxic Nitrates are removed by plants and periodic water changes. (5) Evaporation. (6) Light, (7) Oxygen Cycle, (8) O2 produced by plants, (9) CO2 produced by Fish
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