[Delphinus]

I sure hope wooden shims will suffice. Although what I did on the low side was cut a 2x4 to the width needed and slid that under the 2x4 that was at the bottom of the stand. It was the middle that proved to be a challenge because it was a triangular gap now, so I just jammed a whole lot of shims in the middle under the centre vertical post until they were one solid mass, then I chopped off the bits that hung out from under the stand, and then I encased that in silicone on the tank side and Great Stuff on the outside - those shims are not moving anywhere anytime soon.

I believe Michika used some kind of levelling compound. I would suspect because of the edge cracking you mention, that if you went that route that you have to do an area larger than the tank stand so that the stand is not sitting anywhere near the edges.

But I'm no builder so what do I know..