Pink or Dark Stains in the Toilet or on Fixtures
Airborne organisms are usually the cause.
You will see grey, black, or sometimes pink film on surfaces that are regularly moist, including toilet bowls, showerheads, sink drains, dishwashers, and shower tiles.
These organisms are controlled with normal drinking water disinfectants and, therefore, are not found in the water but can come from dust or dirt that is airborne.
Regular cleaning and ventilation should reduce these nuisance organisms.