Thanks for the info. After reading all the resposes, I was set to go to HD to buy a piece of 1/2 foamboard, but I figured I'd call the manufaturer first to make sure it wouldn't void the warranty. After talking with them (Tenecor), I am now not sure about it.
They said as long as the plywood top is level (and 3/4 or 5/8 thick), they suggest putting the tank directly onto the plywood surface. The reason being, the foamboard could have soft spots, which would cause the surface under the tank to nolonger be level. Water in Acrylic tanks has a habit of "finding" the weak spots in the support surface which would result in a point of higher pressure on the bottom of the tank.
Also he said if one of these weak spots was on an edge, the tank would pack it down and not be level anymore.
This is my first acrylic tank, so if anyone has any thoughts on Tenecor's statements, they would be appreciated.
Thanks again