Get yourself a level, get some wood shims. If the tank is not level insert shims as needed.
For tanks that lean forward, put shims under the front of the stand. For tanks that lean backward, do the reverse. For tanks that lean to the side, put shims along the side it's leaning towards. Obviously, using the whole shim may be too much and may throw the tank off level in the other direction, so figure out how much of the shim you're going to need and cut it to size. I usually level a tank with some water in it so it weighs something and the shims/stand won't shift as you're putting them in.