Salt dip won’t treat what’s in the tank. Gotta treat the fish and tank.I will do a strong saltwater dip on a fish that bad.
OK I've never actually let one get that bad. . . .poor fish.
If your tank won't go hotter you can insulate the top with folded towels or scrap styrofoam. That will help it warm up.
If you have a canister or sump you can do the same. Costs nothing.
Most of the heat lost goes straight up.