This stand does not have leveling feet. Fortunately my floor is level.Just throwing this out there. Is the stand flat on top? If you lay a flat edge across the top does it rock? It almost looks like it may have a high spot in the center. Just wondering if it has leveling feet in the center that are turned out too far causing the center to be raised a little more than the corners.
EDIT: I would not shim between the tank and stand. The only place I would ever use shims is between the stand and floor.
Thanks for the idea and info about not shimming.