What causes a black ring around water in toilet?

Answer 1

Black rings around the water in your toilet, if not a sign of cleaning neglect, can mean hard water in your septic system. Hard water buildup can leave stains that may need a heavy cleaner like CLR, or the use of a pumice stone (wet) to remove.

My experience is the black rings are related to a person with Diabetes. I started dating a guy with diabetes. When he would visit me he would consistently use the second bathroom toilet. The black rings were only in the toilet in the bathroom that he used. This went on for over a year. I recently relocated to another state so we did not see each other for over a month. My new place has three toilets and for over a month none of the toilets had the black rings. Well my boyfriend came to visit for a week. Within two days of his arrival the toilet in the bathroom he used had the black ring at the water line. The other two toilets that he did not use did not have a black ring. I treat the toilet with bleach. I keep a small bottle of bleach under the sink in his bathroom and pour a small bit in the toilet daily when he comes to visit. This pretty much keeps the black ring away. Hope this helps!

Answer 2

Mildew.