You can get a pack of wooden shims at HD or Lowes that is used to level prehung doors. I believe they are 1/4" at the thickest point. Start in the two lowest corners and slowly drive them in with a hammer. If one is not enough, cut the thick end off another shim and insert it under the stand backwards with the thin end facing out, lay another shim on it with the thick end out and drive it in so they are doubled up for more thickness. Keep the shims close the the corners as they are the weight bearing areas. If your stand has a center brace you may want to drive one under the center to spread the load more evenly, just don't drive it in so far as to create a high spot in the center.
Once he tank is level use a razor knife to score the shims and snap them off flush with the stand so you won't even see them.