If you've got rust inside the tank, and presumably that tank has had some kind of coating inside (POR 15, or whatever), I think that means your coating is wearing off, and it probably ought to be stripped completely, cleaned of rust and debris and re-coated. I've used diluted muriatic acid to do this. Muriatic acid is available at Lowes, Home Deopt or any swimming pool store. I'd dilute it around 5 water to 1 acid. Dump the acid into the water, not the water into the acid. Be careful... its nasty stuff... eye protection a must, any a good shop apron will protect skin and clothes from sloshing. Works great on cleaning rust from ferrous bolts or any steel or iron. Do not put aluminum into it - it will eat aluminum. I keep a small bucket of the diluted stuff just outside my shop for cleaning metal parts. I've used a plastic garbage can full to clean fuel tanks of rust and old paint - it will take most paint off of most surfaces! If you just want to do the inside of the tank, you could pour the mix in there and slosh it around for a while (depending on how bad the rusting and pitting is). When its cleaned, rinse it thoroughly with a little baking soda in water (neutralizes the acid) and dry with a blow drier... you're ready to reseal the tank.

No matter where you go.... There you are. - Buckaroo Banzai, 1984.