I had a gap under the tank that I could not get rid of. Every thing was perfectly level so I decided to put down a layer of plywood and stained it. Then I reinstalled the tank and I still had a gap under it except for the coroners. I did some research and found that some people were using the ½ inch foam pad that you buy at home depot. So I bought a sheet of it for about 8 bucks. Brought it home and installed it then reinstalled the tank and filled it with water and 240 lbs of sand. To my surprise the foam pad did not even shrink down hardly at all it is still about ½ inch .It only shrunk a little on the coroners. I saw a couple of huge tanks on reef central that used it and did not have a problem. So I hope it goes ok.