Yes I do know exactly what you mean, but I have a friend that I may try it with. She has some staining on her fiberglass pool. I was going to go over and do the stain treatment with ascorbic acid - maybe I will ask her if it is ok to ty the Proteam to get rid of the stains. I am really interested to see what it would be like, but I don't want to "deliberately" get stains on my pool just to try it either. Maybe chem-geek can look into it to see if metals can fall out as precipitate - I know there are ways to do it with water treatment plants - I was reading about it on the internet - but of course I being chemistry illiterate didn't understand most of it.