Right, so the tank itself is not level, but what you have to do is make sure that the SIDES of the tank BOTH have the same water slope. Meaning when you look at one side of the tank, if it's going in a downward slope towards the front of the tank, the other side should be doing the same thing. You need to look at each side, they should have the same water slope. (One side will obviously have a bit higher water LEVEL, but thats ok, since the tank itself is not level)
Anyway, that probrably confused you even more....do you have a leveler you can use? Just put that on each side of the tank, and the little bubble should be in the same place for both sides. Does that make sense?