Carpet not level?

[Visionist]

Not really sure where else to post this. My carpet is not level in my room, I'm not sure if its my carpet, or floor itself but any fish tank I seem to place in my room is never quite level, this isn't an issue for a 20 gallon tank or similar, but I don't want to put anything larger for fear of it cracking. Pulling up the carpet is not really an option.
I want to put maybe a 40g tank or so at some point.