I really love this tank and hood! but I really don't recommend shimming the tank itself. I would use a 3/4" thick board under the tank and them shim between the board and the work bench. You could then use some trim molding to hide the gap between the board and workbench.
Also, if this is a working workbench, pounding on it will not be pleasant for the tank inhabitants.
Thanks, yeah i knew not to do that, I do have a 3/4 thick plywood underneath it then shimmed that.