Manganese
MANGANESE
Manganese is frequently found associated with iron, but is more rare than iron.
If you remove the lid on your toilet tank and see a red line at the high water mark, the problem is iron. If is black or blackish brown the problem also includes manganese.
Manganese causes a bitter taste in water, and at concentrations higher than 0.1 mg/L causes objectionable dark brown or black stains on plumbing fixtures and laundry. The same equipment that removes iron will remove manganese contamination as well.