Aquarium cycling is the process of growing nitrifying bacteria that convert ammonia (toxic) to nitrite (toxic) to nitrate (relatively harmless). Standard fishless cycle (3-6 weeks): dose ammonia to 2-4 ppm. Wait until ammonia drops to 0 within 24 hours, then nitrite drops to 0 within 24 hours. Cycle is complete when both readings hit zero in a day and nitrate is rising. Accelerated cycle (1-2 weeks): add seeded filter media, gravel, or sponge from a healthy established tank. Or use bottled live nitrifying bacteria (Dr. Tim's One and Only or FritzZyme 7). Both contain real Nitrosomonas + Nitrospira; many other "starter" products do not. Saltwater specifics: live rock dramatically accelerates cycling - often 1-2 weeks total. Dry rock + dosing requires the full 4-6 weeks. Signs cycling is complete: 0 ammonia, 0 nitrite, detectable nitrate (5-20 ppm), stable for 5-7 days at 2 ppm ammonia dose.
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