Best thing I ever did (and my tank is on carpet, but in our basement) for my large tank.
Went WITH MY WIFE and purchased (from home depot) a piece of lineoleum for under the tank (with about 4-"6" overhand all the way around, and a 2nd 4' x 4' piece (but my tank is 6' wide) to put down when doing water changes.
As for the carepet we had 125 upstairs on carpet and there still is a slight indendt 3 year later (but I was told a trick recently to get rid of it, but since we are replacing it we are not going to do).... involved wetting the carpet and get a strong vacuum.
Dave