avoid reptile sand as most would just break down overtime, and is probably not healthy for the water at that point. Dry argonite, which PetStupid carries would work just fine. As in time, it will become live anyways. You could collect rock from the rock shore, but you want to be very careful to fully cure, kill, cure, kill and re-cure again, to kill any negative hitch hikers and other things you may not want in the tank.