, I found one wall in my house that was actually plumb when it was built.
If the tank is sitting on a wood framed floor, the floor is probably sagging or the stand is not built square. If it's the stand, shim it to sit plum, surface level and flat. If it's the floor I'd look at supporting it with additional framing. Use a 3' or 4' carpenters level to scope out level-a-tude. A carpenters type square to check if the stand frame was built square.