I am getting ready to close the pool, and have drained the water past the skimmer. I have about a 6in dark scum line around 3/4 of the pool above the initial waterline. I have read that magic eraser is good to use. I have bumped up my chlorine level to 11ppm to prepare for closing. Any thoughts on magic eraser? Is it ok at 11ppm to use? Thanks for any replies