What causes the grayish-black ring in the toilet bowl?
The grayish-black buildup that is observed in toilet bowls is the result of fungal growth. In many cases this growth is composed of several different fungi and other organisms. The source of the fungi is airborne fungal spores. These microscopic spores can be dispersed throughout the house by air currents. When the house is vacant (i.e. Long weekends and vacations) the fungal spores are able to rapidly multiply in the toilet. The water in the toilet is open to the environment and rapidly loses its chlorine residual. The internal area up under the lip of the toilet provides a refuge for the fungus. It is from here that the bowl is re-inoculated after cleaning. In most cases, growth reappears within several days after cleaning. Routine cleaning provides only short-term relief. The bowl and the tank must be disinfected after cleaning. Adding Clorox to the tank and bowl can do this. A contact time of a half-hour or more must be allowed. This procedure might have to be repeated several time
Here is a clip from the municipal water authorities in upstate Buffalo NY.
Why is there a black ring inside my toilet bowl?
This has nothing to do with water being delivered to your household. This is mildew caused by
room temperature, facility usage and lighting. We suggest a chlorine-based bowl cleaner, and
more ventilation during showers.
My situation is as follows;
This was the case in my home, the extractor vent for the bathroom was ineffective. You may have closed windows, poor ventilation etc. In my case, a little investigation found that this extractor vent fan, vented into the attic an not out to the atmosphere (poor construction). As a result we stop turning on this extractor fan to prevent mold and mildew buildup in the attic. So, during showers and toilet use we got this mildew build up from moisture in the toilet. See, there is a REASON for these ventilation systems in homes with windowless bathrooms. (town-homes, apartments, houses etc).
Improve you ventilation and you will eliminate this problem.