Depends on what type of tank. Fish only will use the Bio-Bag in the filter. If its live rock/live sand, then just place rock and sand in tank and monitor the water. Usually die off on rock will produce ammonia to start cycle. Fish only has two options, fishless cycle with ammonia as previous posts in this website have havew written about, or place hardy fish, such as Damsels in tank. Live rock setup will have less of a stress on fish due to nitrifying bacteria already existing in the rock. Fish only will have to start from scratch.