I set my tank on tile floor as well and was able to get it to line up on the grout lines (which were pretty level.) The tank is really just supported around the edges. If it needs some adjustment I would consider using wood shims on the edges that need support. Wood is the best choise IMHO because you can easily sand it down in the places where it is too high. Just be careful to support the entire frame as much as possible and not just prop up the corners.

Alternatively, if there's just a high spot or two created by a tile edge then you may just be able to use a file to take a little chunk out of the frame so that it sits level.

Good luck!