Shimming involves placing spacers of some sort (wood shims are very common for this application, but I prefer 1/8" aluminum barstock) to level or plumb the junction of two planes. The bottom of your stand and the floor, for instance.
I have tanks that are less level than the 55g that you're describing, and as such I wouldn't take the risk try to shim an active tank.
I have tanks that are less level than the 55g that you're describing, and as such I wouldn't take the risk try to shim an active tank.