You do not introduce animals until after the cycle, which may take from 4 to 8 weeks depending on the quality of the rock or the method you're using. My own, that takes about 4 weeks, involves dropping in a pinch of regular flake fishfood and waiting until there is ammonia, then no-ammonia. Follow that with a 4 week quarantine for your first fish, or dipping and observation of your first coral; but meanwhile, give it about 4 weeks of ONLY micro-hermits, snails and bristleworms---your store may GIVE you a few of the latter. They reproduce fairly well in the average tank. Once the snails have done their thing for 4 weeks, the tank should be stable for one fish.