If the tank is really rusty or something then you should just get a new one. If you are looking for a mild clean then you can use some sort of solvent to take of some grime. If you are looking for a hardcore clean then you can try muriatic acid but this stuff is toxic. Other option is take it to a shop. They'll probably fix you up for a couple hundred bucks. The easiest way to tell if there is rust in the tank is just by checking the pump. If there is rust on the pump then there is rust on the tank but if there's nothing then I wouldn't worry about it.