You can buy a pack of synthetic shims at HD for under $5. I forget the brand, but they carry one that once you shim it under and the tank is level, it snaps off at the end so you dont have the long piece sticking out from under the stand.
Shim under stand, NOT tank. If, by placing shims in one area, other areas of stand are not making complete contact with floor, shim those areas as well. Entire bottom of stand must make complete contact with the floor, even if this contact is made through the shims.