I sanitize it each trip unless it's within 2 weeks & temps haven't been too hot. My white, transluscent fresh water tank is visible by just looking under the camper. A few years ago before I started this practice, I looked at it after about 3 weeks of hot weather.The inside of the tank was green with mold. I had drained the tank after the previous trip but there is no way to actually dry it out so it's damp in there & mold does form whether you can see what's going on inside your tank or not.