first, southdown is 50 lb bags, so that is an even better deal, yes it is worth it.
2d, what are we trying to clean up or off? what water stains? where? inside the tank? I am confused how we get water stains inside of a full tank. help me out, so i can try and help you(if possible) BUT I would not use any cheems(safe or not) and vinegar if I did not need to. BUT definitelyno chems!