Nitrogen Cycle

The nitrogen cycle involves two primary bacterial communities: Nitrosomonas species oxidize ammonia (NH₃/NH₄⁺) to nitrite (NO₂⁻), and Nitrospira/Nitrobacter species oxidize nitrite to nitrate (NO₃⁻). Ammonia and nitrite are toxic even at sub-ppm concentrations; nitrate is relatively harmless below 40 ppm in most freshwater fish. 'Cycling' a new aquarium — establishing bacterial colonies on filter media before stocking — takes 4–8 weeks under normal conditions. Ammonia dosing or fishless cycling with pure ammonia solution accelerates establishment. Live rock in marine aquariums provides a vast surface area for nitrifying and anaerobic denitrifying bacteria, creating a near-complete nitrogen cycle within the system.