Not sure how to explain, hmmmmm, lets see.
We fill the tanks sometimes, there is soap scum and gunk in the tank with this water, it floats around, but some of it does attach to the walls and floor of the tank.
Sometimes we leave the valve open, what was wet now has a pretty good chance to dry into nice concrete stuff.
When we let the tank fill again, the stuff is so dry and stuck to the walls and floor that it may not rinse out.
We now add a little more, well, you get the idea - - -
The bad part is that when the tanks are open, valve open, that there is not enough water pressure to clean out any particles and soap scum that accumulate at the bottom of the tank. When we allow the tanks to fill up, the volume of the water makes a great deal of water pressure which really helps rinse out particles of stuff in the tanks.
We worry more about the stuff that is stuck at the bottom of the tank than what might be stuck to the sides and the sensors. Since we don't use the sensors anyway, we are more worried about accumlated "stuff".
Just a thought here, we hope that the Calgon water softener trick will loosen up any of this gunk and wash it away.