I am in the process of closing my pool and would like to get rid of the stain. The stain is light brown and along the bottom perimeter of the pool. The stain has been there for a while, I also had iron stain problem which was resolved with ascorbic acid treatment, but the light brown stain is still there. So I am assuming the stain is organic. In preparing for the season closing, I've shut off the pump and plugged all the returns. I am shocking the pool and have increased the FC to above 20, all my readings were normal prior to closing, CYA was about 40. It's been about 5 days, I was able to maintain the FC day and night without having to add too much chlorine. I have an automatic pool cover so the pool is covered almost all the time, I tried to brush the stain twice a day. So far the stains are still there. Should I continue my shocking and brushing regime? Will the high FC damage the underside of the pool cover since it is in contact with water?