Tanks without trim on the bottom need to have styrofoam between the steel and the glass. 1/2" rigid does just fine, but you can go thicker if you want.
People often recommend plywood under tanks with no trim, and tanks that do have trim can be put straight on top of a steel stand. I'm not sure why people have come to the habit of making this recommendation. Tanks with trim don't have the center supported, the glass is raised up, and the tank is only supported on the outside perimeter. As soon as there is no trim people start recommending a flat surface. I'm not sure that there is any need for that. HOWEVER, if supporting the bottom glass in its entirety (using plywood + styro) is an option, I would certainly do it.
Make sure when you install the tank and stand that the tank doesn't "rock" on the stand, and the stand doesn't "rock" on the floor. Steel stands warp during the welding, and some welders don't correct that (some just aren't as picky as we'd like them to be haha).