Since switching to the BBB method a last year I have noticed a distinct discoloration of my liner below the water line and am not sure why. My PH level is normally 7.2 - 7.5. It may be out of wack for a day or two, because Im out of town but I try to keep it stable. My CYA is 40 - 50 so I try to keep my chlorine level around 5-6ppm and shock as close to 15 as I can get, but usually only need to shock once a week at most.(usually after a weekend of family swimming) What would cause the liner to fade like that?

Also I always have my solar cover on when not in the pool, I leave it on when I shock as well. I have notice that the cover is becoming brittle and I have read that this could happen. Could leaving the solar cover on when shocking be causing the discoloration of the liner? I just lose too much heat when its off.
Thanks for any advice
John