Not sure this answers the exact question.... but the "big stores" sell a shelf liner for closet shelves (the white metal ones you usually install) that is a very dense (about an 1/8 inch thick) foam liner for cheap (5 or 6 feet (by one foot) is just a few bucks). It can be cut into 1 inch strips to go under tanks or anything. I put it under every tank I set up. I'm not sure it is the cure all, but I don't have leaks and everything seems to stay quiet....
Just my $.02 worth,
johnny