This film can be a result of many factors, some internal to the home, such as a water softener or plumbing. It may also be related to the condition of the water coming into the home. Hard water can leave deposits, which are the mineral salts left behind as the water evaporates, on toilets and dishwashers. Rings on baths and showers can also be scum left behind as the water evaporates or from soap or shampoos reacting with hard water. (NOTE: Methuen municipal water is NOT hard water, therefore this is an unlikely reason). Black slime is usually mold/mildew that thrives in moist areas like bathroom toilets and tiles where it is wet and warm. The film that develops on sink stoppers is again non-harmful bacteria and residue build up. Usually, the customer will need to clean the area with a commercial cleaner that contains a disinfecting agent, such as chlorine bleach.