fishfishfish. You don't need foam under your tank and you don't need it in your tank. If you want to keep the rocks off of your glass without having a substrate between them then I would suggest you use eggcrate under your rocks. I'd be worried about diggers (cichlids) tearing off chunks of foam and having them floating around.
Everyone else. Think of it this way...if a piece of sand gets under your tank and your tank's bottom glass sits flat on the stand then your gonna have problems. If you have the plastic frame then even a piece of gravel shouldn't cause a problem as it will not touch the glass. I do not condone placing gravel under tanks btw. the sand and gravel is just a metaphor of sorts. The grain of sand could be anything, an unsanded spot of wood, a gradual bow caused by moisture, a nail, anything really.
Everyone else. Think of it this way...if a piece of sand gets under your tank and your tank's bottom glass sits flat on the stand then your gonna have problems. If you have the plastic frame then even a piece of gravel shouldn't cause a problem as it will not touch the glass. I do not condone placing gravel under tanks btw. the sand and gravel is just a metaphor of sorts. The grain of sand could be anything, an unsanded spot of wood, a gradual bow caused by moisture, a nail, anything really.