Our toilets have recently begun developing dark brown, even blackish water lines coming down from the rim where the water pours into the basin. We clean regularly and I cannot recall this phenomenon occurring before. Could it be a water issue? Should we clean the tanks?
You have a lime-like mineral build up from the many little holes under the toilet bowl rim where the water flows during flushing. These water paths have left a mineral residue over the years and that residue is now holding moisture where mildew can form. The black is from the mildew, mildew is from the wet mineral deposit and the mineral deposit is from hard water over the years.
The solution is to clean your toilet bowl with a scale or lime removing cleaner. You can use acid, household vinegar, or a household cleaner to remove these deposits. The household cleaners will have some form of acid in them and have a pungent smell … like Lime Away. You can use the acid if you want to be quickly effective or choose a household cleaner like the CLR, Lime Away etc. This is an easy solution.