Thanks for the quick response! I did review the article and did read all 5 pages of message thread related to the ascorbic acid treatment. The stains became almost black in some places immediately upon adding the acid, on the sides as well as ledges. Brushing had no effect, they lightened up just a bit in the ensuing days but they still look hideous. Although the acid did not remove any stain, I did add a bottle of sequestrant after the second dose of 5 lbs. The ph was 7.6 when I began, it went down to 7.2 after the first treatment but remained at 7.2 after the second treatment.Why would the PH not budge after adding that much acid? I have never had an algae problem, am pretty diligent about maintaing the chlorine level, free cholrine is usually ok and with the nature 2 I would have thought there should not be an algae problem??(I do realize if in fact the stains are metal, the nature 2 would be contributing and would get rid of it). I saw a picture in another post, the problem was identical but I did not see where he had resolution. Organic stains would come off with enough chlorine and brushing, right? I cant seem to find information on stains other than those 2 topics.