I've a recurring stain problem in my fiberglass IG pool. Tried all of the recommended treatments - work fine but within a week the stain is returning even with above recommended doses of sequesterant. I'm currently using Pro Team's Metal Magic as it will remove the stain without going thru the ascorbic acid procedure (which I've done 3 times).

Water supply is city water from sourced from the Great Lakes so the metal isn't coming entering with fill.

I'm wondering, the heater is 10 yo. - Is it possible that the heat exchanger is corroding and could be the source of the metals? Putting metal into the water at a faster rate than I can remove it or keep it suspended?