Manganese? Can you look back at a well water test? If not get one done. Go to your local plumber/ plumbing store, they'll give you a bottle to fill and they'll send it out. Or a spa retailer who can test for metals in your source water.
If it's iron, if can be sequestered using stain and scale control(BioGuard product), or others like metal out, metal gone.
Add 1/4 cup initial dose. then use two to three capfuls per week.
If it's manganese, most sequestering type chemicals do not help. Good luck, let us know how you make out.