Disinfection of non-potable water?

I have installed a 550 gallon black plastic water tank to provide water to a cabin. The tank will become very hot in the summer sun. The water will be used for bathing, washing, flushing, but not for drinking. It has been suggested that this might not be a good idea because of Legionella. Could I add anything to the tank water to help? It has been suggested that I add fresh water on a regular basis. The source is a creek that is fed by springs in the Texas Hill country.

Place a warning that the water is not for drinking. You could also disinfect the water with chemicals such as bromine. In addition to bromine, a small amount of bleach (free chlorine) can be poured into the tank for residual disinfection. You should calculate the amount of bleach added. The final concentration of free chlorine should not exceed 1 mg/L.

The idea of adding fresh water into the tank is to provide residual chlorine from the “fresh water” into the tank as disinfectant. If you use the creek water, then it will not solve your potential problem.