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?

[Brent F]

Since the frame of the tank supports the bottom any foam in the centre wouldn't touch the glass. If you do use a foam it would only need to go around the outer edge since the bottom glass won't be touching it.

I'd be concerned that the framed tank would break away the edges of a styrofoam put under it since all the weight would be on the edge of the foam.

If you do decide to use a foam, I'd use a closed cell foam and wouldn't use anything thicker than a 1/4 inch (1/8 would be a better choice).

[Pier Pressure]

I had a 150 gallon blow up on me twice in eleven months. Consequently sold all my freshwater. I did not have anything between the tank and stand and it appeared to be flush - and I was informed that if I would have had something between the tank and stand it would have voided the warranty. I ended up getting my money back, but be sure to check your warranty. Tank and stand were matching and appeared to be completely tight to each other, and the stand was tight to a stone tile floor. Just FYI.

[fkshiu]

I had a tank crack WITH a piece of foam in between stand and tank.

[Doug]

I always used foam under my custom made no frame tanks, so the glass sat on the foam. I have never and would not use it under my framed standard 90g AGA. I would check on voiding warranty or not, if using foam under a similar aquarium.