Xort,
I also have a short piece of clear tube between the fresh water
pump and the pvc quick connect water pipe that runs throughout the boat. I have done as you suggested and wrapped the 18" length with black duck tape to keep the light out. I have a strainer between the tanks and the
pump that has a clear plastic cover. I find that this lets me see what's going on with the water -- when the fine mesh strainer gets full of goo, it's time to flush the tanks. When it starts getting clogged with bits of rust and
paint chips, it's time to get out the chipping hammer, and sand paper and
paint the inside of the steel tanks.