In my opinion the idea of styrofoam "leveling a tank" is a myth. The compression strength required to compress styrofoam (dense blue/pink or more pliable white) is not met with a tank even if it is off level and very large. The weight is spread out evenly so the tank will remain off level and the tank will be as level as the stand below it.
I use 3/4" plywood to spread the weight over the stand and support the bottom to minimize deflection. tanks with holes in the bottom are particularly weak in this respect. With glass tanks I use 1" blue styrofoam over the plywood to preserve the tank warranty, but I only do it to make the manufacturer happy.
The only benefit of styrofoam under the tank is to guard against imperfections in the stand or plywood directly under the tank. As sphelps has suggested, rubber works just as well and it's cleaner with a lower profile. It's impervious to solvents and physical abrasion. In the end you have to do whatever it takes to make your tank manufacturer happy so he honours the warranty.
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