You do not mention where the boat is. In Florida, we Keep our tanks full during the summer months. First, it adds more weight to the boat to help stabilize it during our stormy weather. It also cuts down or eliminates the formation of any growth inside the tanks. We also add a cap or two of bleach. No residual order or taste.

We store in a frozen land, so we have to empty our tanks and lines or risk cracks.  We have no need for the full 80 gallons of fresh water, so leave the forward tank empty at all times.  I'm not a water expert, but I would think that empty and dry is less likely to grow anything than full of stagnate water.