Depends on what type of rock. Fiji rock is very porous so most say 1 lb per gallon of tank volume. If you are adding sa and bed you can get buy IMO with 1/2 lb per gallon (do to the amount of bacteira that can grow in a sand bed) If you are getting Tonga mix it with the Fiji because tonga is very dense and does not give you the benifit of growing bacteria use it for structure building.