One of the Reasons I Don't have a Sump

[rusty]

The foam will support the tanks actual glass bottom and if the ply warps or does anything else funny the foam will absorb it.

I had a tank that sat on plywood to support the bottom there was no trim on the tank so the glass sat directly on the wood. The wood warped and the tank cracked I really like having ridgid foam under the tank. Muck used some really good foam under his tank http://www.canreef.com/phpBB2/viewtopic.php?t=11702
I really like this stuff its soft enough to absorb irregularities in the wood but firm enough to support the tank