What can I use to kill germs in a metal tank we want to swim in?

It's probably a five hundred gallon galvanized tank. We can change he water but it's a lot so we don't wanna do it to often. So could we use bath salts like epsom (Maganese), or borax, or vinegar to kill germs between changes? I don't want to use cholrine or bromide because it is reactive to metal. But if I did how much would I use for that much water?