styrofoam

[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).