styrofoam

[Aquattro]

Typically people put carpet, insulating foam or some other mildly compressible material under the tank. This just gives an even surface for the frame to sit on. I've seen tanks without it which are still full of water, but I've always used something. Right now I have a really thick type of underlay under mine.
styrofoam would probably be too "puffy" and compress too much. Dunno, nevr used it.

Jason, go to HD and get some of that blue or pink 1/2" foam, works great. Or don't, but we don't want another crack, right?