Hot water heaters frequently have sacrificial anode rods to prevent corrosion. The rods corrode over time and dissolve into the water. Lower cost aluminum rods have health concerns, not as much with magnesium rods.

See also Tom Scott's video on Britain's history with separate hot and cold taps and why hot taps sometimes had unsanitary cold water tanks to supply them: https://www.youtube.com/watch?v=HfHgUu_8KgA