I have the XL425 and I only shimmed it in the front and it has been fine for 5 months now. I used the composite shims from Lowes. Got it all level when empty, filled with tap water, let it settle for 48 hours, made sure it was all still level. Then I drained it and cleaned it, and filled with rock, sand, saltwater. I have mine about 4 inches off the wall so it was away from the tack strip on the carpet and to be able to get to the back if needed. Because of how they sit on the stand, the reefers seem to tend to lean forward a bit, so chances are you will only need to shim the front, if anything.