You will need to shim up the side that is lower.
-set your level on your tank.
-raise the level on one end only, until the bubble is between the 2 lines.
-once the bubble is between the lines, there will be a gap between your tank and your level on either the left side or right side of the level
-if the gap is on the left, the left side needs raised (shimmed)
-if the gap is on the right, the right side of the tank needs raised (shimmed)
- you will want to do this same procedure going from front to back of the tank
-once the tank is level in ALL directions, place shims every 3 inches under your stand where there are gaps. This is key in order to keep your tank and stand from failing due to sagging in the middle.
Hope this helps.