I too have a 21,000 gallon fiberglass pool and a salt to chlorine generator. We have extreamly hard water and I just had my generating cell replaced after only two years. (free of course) Last year I began to have brownish 'build-up" at, or above the water line only in two areas (both wind driven areas) I had been using a 'Bio Guard" product "Off the Wall" on a weekly basis until late last fall. This year the stains are quite noticeable and no amount of "Off the Wall" and/or elbow grease seems sufficient. Do you think the Ascorbic Acid trick will work for me? Remember that most of the stain is above the water line. Will scrubbing with the ascorbic acid do the trick.

By the way I life in north western Arizona.

Ralph