It is just common sense to use foam under a tank. I assume you sleep on a mattress; that is the same principle as the mattress removes pressure points on your body. If the tank and stand are flat, and nothing changes over time, then it does no harm. However, any one of the six factors below causes tank stress that can be minimized or eliminated by placing foam under a tank: 1 - The four corners of the stand are not even (i.e., one corner is higher or lower than the other three). This is very difficult to check. 2 - The tank stand does not provide adequate support around the outside perimeter and center of the tank. 3 - The stand is made from wood and any part of the stand compresses or sags over time (very common with particle board stands). 4 - The stand has a sharp high point from a nail or knot in the wood that puts pressure on one spot in the tank's bottom frame. 5 - The floor that the tank sits on is wood and it sags unevenly over time. This is very common and often unnoticed as it takes months for the wooden floor joists to start to bend under constant pressure. Seasonal changes in humidity levels also causes wood floors to shift. 6 - An uneven concrete floor will cause a wooden stand to slowly bend and sag over time. So you have a choice: - use foam as an insurance policy and lessen the chance of a tank failure along with the loss of all your fish (tank manufacturers are not responsible for fish loss). I can't recall anyone on this, or other fish forums, mentioning a manufacturer's refusal to replace a tank that had foam under it unless the foam was not under the frame and pushing up on the glass. - or don't use foam, experience one or more of the above six, tank breaks-fish die and the manufacturer replaces it.
