Water tanks and any additives?

A hello to all the nomads from Mittelerde! With WW it was/is it easy … the tank has to be filled up regularly because small and the water in it is always fresh. With the campers this is already a bit different. In the tank already fit, depending on the model, around 70 liters. If you don’t shower the water remains there already a few days. Are there any (legal) tricks or means that can be added without there being a chemical broth? Greeting from the southern states