I had small stains in my fiberglass pool for a few years. Asorbic acid removed them but they came back. So I bought a filter I think called Metal trap that claims it removes iron and copper from city water. I used it to top off in the spring, about 2,000 gal in a 10k pool. Iron went from 3 ppm to 1 ppm. Now I have no stains. BUT don't count on this for your situation. I don't know why my stains disappeared, just happy they did.