Actually the crushed coral is lighter and will stay at the top. At least that has been my experience. I would stick to the 1/2 inch method, that Chris mentioned, or at least no more than 3/4 inch. It will kill off some of the deeper critters if you add too much. You also need to consider the cucumber can be killed by crushed coral. They try to ingest it and it gets stuck, and cuts them up inside. Oh, and don't stir up your old sand too much it will put a lot of nasty bacteria in the water that can kill the fish.