The most common Marine holding tank sensor is an internal tank float of some kind. Because they are mechanical, they tend to foul and stick, especially in a saltwater environment. One tugboat club in Washington did a test on identical tank sensors. After removing the sensors from the tank, they hand cleaned the first sensor. It required some effort and stiff brushing to get it clean and moving freely again. The second sensor in similar condition was placed overnight in a 5-gallon bucket with Forget About It For Boats added to the water in the bucket. The next day it was removed and the crust that enshrouded and blocked the posts was washed off with just hose pressure and no brushing. We do not know how typical this test results were, but we do know that adding Forget About It For Boats will do the same thing in a holding tank. The wave motion in a moving vessel will have the same effect, cleaning the sensors in the tank plus it will keep them clean and the tank odor and sludge free.