One of our bowls develops a dark matter formed at the water line in the bowl made up of discrete "pieces". It flakes off easily but reforms again in a short period of time. Other three don't do similarly. City water instead of well. It's a Mansfield while another down the hall is American Standard but I can't imagine porcelain difference, if any, being the cause. Ideas?
First guess would be flapper valve (or other rubber parts in valve, etc.) deteriorating and you're seeing the floating debris collecting at the water line. If water is high in chlorine from treatment, this is a fairly common problem ime...
Yep, check the tank to see whats going on.
Could be that is the one frequently used by one of the kids who does not always flush the urine? It may not be very noticeable, but if not flushed, urine contains a lot of solids and it can build up over time.
Is it in the line someplace that minerals in the water would tend to collect?
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