In mine I placed all the rock in the desired formations directly on the bottom glass. Call me stupid, but I did so, so that the fish wouldn't be able to dig the foundation from its delicate balance. Once this was accomplished, I then placed the sand around the rest of the area and haven't looked back, plus I can peek at the fish from within the stand in the lower caves I made. I have 130lbs of lava rock in my 80/90 gallon tank.. All I really have to worry about is the dreaded "big one" California has been expecting for the last decade.. (knock on wood) wouldn't worry about below it rather the front pane.