It's good that you have no water in the tank.
Tanks need to be supported evenly all the way around. Shimming directly under the tank is not perferable, but if there is no other way, then that'll have to do.
The first thing you need to do is pull the tank off the stand and use a level to make sure that the stand is completley level in all directions. If not, shim between the stand and the floor.
Once the stand is leveled, put the tank back on, and I'll bet your problem is solved. If not, your stand is not true. In that case, the best solution is going to be to put a piece of foam insulation under the entire tank. That may even out slight differences that is causing the issue. If that doesn't work, shim between the foam and the tank.