I understand your logic. I can only go by my experience - I tried logic with the stains, but it got me no where. I then decided to figure out what was the best way to keep my pool stain free. I tried to find out where the iron was coming from - my water tests that there is no iron in it. I still get stains.... So I just treat the water at the beginning of the season - with metal free before I add the chlorine and balance the water. If I get stains, I treat them, but I find that if I keep my water balanced so I don't have to shock, or run high chlorine levels, I don't have problems with stains. So this was just my advice - I don't promise that it is right, or logical. I just gave my experience on what I did, and what I thought was the best and cheapest way to go.