Aquarium cycling (the nitrogen cycle) is the foundation of every functioning aquarium. What happens without cycling: fish produce ammonia through gills + waste. Without bacteria, ammonia accumulates rapidly - 0.5 ppm is stressful, 2 ppm is lethal within 24-48 hours for most species. The cycle: 1) Fish + waste produce ammonia (NH3/NH4+). 2) Nitrosomonas bacteria oxidize ammonia to nitrite (NO2-) - still toxic. 3) Nitrospira bacteria oxidize nitrite to nitrate (NO3-) - relatively harmless until 30+ ppm. 4) Plants/algae consume nitrate, or you remove it via water changes. Where bacteria live: primarily on biological media (ceramic rings, sponges, bio-balls, K1, live rock) - 90%+ of the colony. Smaller amounts in substrate + on glass. The water column has very few. Why "old tank water" doesn't cycle: bacteria live on surfaces, not in water. Save filter media + substrate from a healthy tank to seed a new one. Mini-cycle triggers: replacing all filter media at once, antibiotics that kill bacteria, deep gravel vacuuming in immature tanks, big stocking jumps. Test cycle status: dose 2 ppm ammonia, wait 24h. If both ammonia + nitrite read 0 with detectable nitrate, you're cycled.
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