The stand is in the basement on thin carpet and concrete under the carpet. I got the 1" thick pink foam. I'd rather have too much than not enoughA few thoughts
is the stand on carpet or hard flooring
the weight of the water in your other tank may be deflecting your stand and when you add the water weight of the new tank it might straighten out
I don't think that little of a gap would be a problem with a thin layer of foam