Levelness of a tank

nothing to worry about. Mine is as well. I did, however, anchor it to the wall studs because I have small kids. Depends on the size of the aquarium i.e. even 30 gallon tanks are 300lbs, kinda hard to move!
 
Its hard, if not sometime impossible, to get a tank perfectly level. The danger in a tank that is not level is an uneven load on the seams of the tank that could lead to a leak or failure in the seam.

Unless the tank is huge or noticeably out of level when you stand across the room from it, you should be fine.
 
AquariaCentral.com