What kind of stand do you have?
If your stand is indeed causing it to be that uneven I would suggest either getting a new stand or finding some way to fix it, because if there is a weak spot in it somewhere it could collapse. The last thing you want is 55 gallons of water and fish on your floor, I'm sure.
rofl I keep my 55gal on a sturdy wooden set of drawers that two 300lb men could tap dance on all day, and even that makes me nervous sometimes lol. (Then again, I'm a very paranoid woman.)
Anyways, find yourself a leveler so you can see exactly what is causing the tilt. You can probably find one at a hardware store if you don't have one.
It's a neat little tube of water with a bubble floating in it. Looks something like
this.
If you find out it's your floor that isn't level, well take Prince Sushi's advice and prop one side up. Make sure it's as even as you can get it. Only do it if you're 100% sure that whatever is propping it up can't get knocked out by accident~
Good luck with figuring out what the problem is!