Tips to get stand level.

[hillegom]

I'm not quite sure where the problem is exactly, Is the wood not level or the tank is not flat on the bottom. Or is the plywood not sitting on the steel stand.
Regardless, don't use cardboard. It will squash flat and not be of enough help to level. Use something that is solid that won't squash.
How much of a gap are we talking about? If just 1/16. Use a pink styrofoam sheet and it will squish in the right places to level the tank