I have gotten rid of rust in tanks before by filling them with white vineger and letting them set a few days, giving them an acasional shake every day or so. Once its rust free, I run a garden hose in it for 5 mins to flush any particles out. try to get as mutch water as i can out, then shake some gas/2 stroke oil mix in it, dump that out and fog the tank with wd40(dont be stingy.....Doing this should leave you with a new looking tank. Good luck!