grey tank

The problem is not with scum on the sensors but with crud on the tank walls that is wet and fools the sensors. This proven by the fact that the same problems exist with external sensors that are on on the outside of the tanks. They usually work with more crud on the tank walls than the through the wall sensors but will still give inaccurate readings if the crud build up is bad enough.
Anything you can do to clean the tank walls may help especially if you are a weekend user and not a full timer or extensive user. As a full timer for 11 years I never found anything or any product that worked (I had external sensors). I ended up just marking the calendar and dumping every 10 days.