I was just researching this myself. It seems a lot of people line the floor with egg crate to help with rocks slipping out of place and possibly destroying fish, corals, other rocks. Some people stack the rocks and others glue them together. Still others drill through them and then use a thin rod of acrylic from Home Depot to "skewer" them. Secure the rod to the floor of the tank (or egg crate) and the rocks will never be able to move. Done properly, you won't even be able to see the acrylic rod.