People typically use styrofoam under tanks to fill in tiny gaps between the top of a stand and the base of the tank. The thinking is that it'll prevent pressure points and create uniformed pressure on all areas of the base. Manufacturers disagrees, and will void a tanks' warranty if they find out you've done this. However, in the scenario with the metal rack flexing or contorting, styrofoam/padding will not be of any benefit.....it'll make matters worse if anything.If you are worrying about flex, you could always put styrofoam insulation under the aquarium.