Floor support??

[Troy F]

Are the joists hung or sitting on the top plate?

Here is a suggestion; set up the tank in the place you want it and check the level with it empty. Then fill it with water and let it sit for a while and check again. You can then estimate what you'll have to shim. {If you come back and the tank has been replaced by a hole in the floor I would advise choosing another spot :D .}