[wwer]

Already been done. Along with alum, tons of it used in India for decades to purify water. Various case studies that list benefits of it.

[AstralStorm]

Too high doses of manganese are clearly linked to Parkinsonism. (If you want, I can list the sources.) That however pertains to acute intake, not chronic.

To get manganese ions, you'd have to acidify the water somehow too.