I HAD SAME PROBLEM ,I TOOK A ASPO OR RUST INHIBITOR IT KILLS RUST AND TAKE VALVES OFF TANK AND LET WATER HOSE RUN THREW SEVERAL TIMES THEN CLEAN WITH A DEGEREASER AND YOU CAN BUY SOME TANK SEALER THAYT YOU POR IN AND IT COAT TANK HAVE TO LET DRY 96 HOURS BUT IT WORKS I HAD BIG CHUNKS OF RUST IN THE TANK,MIGHT TAKE LONGER THAN YOU WANT BUT BE PATIENT