Good thing someone clarified that you shim the cabinet & not the tank.
Just grab some shims from Home Depot, if you have a paint scraper or a flat bar that you can turn backwards, it will make tapping in the shim easy and will help prevent any damage to your floor.
Before you go shimming the cabinet, you should make sure it's not the cabinet that is settling. If it's particle board, has it gotten wet? Has it gotten wet & you let the water evaporate rather than wipe it up? Once particle board gets saturated, it will dry but it will lose it's strength and slowly collapse over time.
As long as the cabinet & countertop are level and not tweaking, it's not the total end-all if it's slightly unlevel, but you definitely want to level it out and find out if it's your flooring or cabinet.