It's a perfecto tank. If I were to use styrofoam, what thickness should I use?

Depending upon teh construction of the tank, of course, since some do
want support and some don't.

If it's a tank that needs support, leave a cut-out around teh brace -
the whole point is o avoid concentrating stress, and from the
description, it sounds like you'd have one at the brace if you just
put it down on a continuous layer of foam.

regards, Ian SMith