Placing a thick pad under an aquarium will help ease the stress on the glass or acrylic and avoid the cracking of glass tanks that several members have written about. In addition, wool is a good insulator and will help to maintain the tank at an even temperature.
Sutherland Felt Co. in Michicigan sells an excellent material called Eco-Felt made of genuine natural wool, which comes in various thicknesses, from 1/4 to 3/4 inch, formed in sheets 60" wide.
Natural wool felt is expensive but worth the cost, since real wool will last for years and will not become brittle like plastic felt, or styrofoam. Check out the website www.sutherlandfelt.com
for complete information about the products. I recently ordered a 60" x 36" piece of Eco-Wool felt and the service was prompt.