Maybe, but if the plywood serves the same purpose, I don't see the point. Not just saying that to save money, I know the foam is cheap.
Okay here is his reply
Do not use styrofoam or rubber, it is a petroleum product with chemicals that will slowly eat away at the bottom panel of your tank, other customers have done this without asking and approx within 1 - 3 yrs the tank fails
Like I told you rest the tank on 3/4" plywood only (no screws in the plywood - leave it loose)
the plwyood plies will normaly crush and level off when you fill the tank, it's meant to ..