I just set up a 230 in the basement using cinder blocks. Most basements aren't level, they aren't supposed to be. They should be sloped to the floor drain.
I was wracking my brain as to what to use for shims as I don't like wood. If the wood gets wet and does not dry properly it can rott and due to the nature of a fish tank I figured my wood would stand a pretty good chance of getting wet a time or two in the life span of my tank. LOL
What I came up with was shingles, regular roof shingles. They are fairly water resitant and will only crush down so far. It's basically gravel stuck to paper using tar, even if all the paper dissolved you would still be left with gravel and tar. That is my thinking anyways.
Oh and I also placed my tank on a sheet of 1" foam, between the tank and the plywood. Foam will help level a tank to a degree.