I am currently shocking my pool due to excessive FC loss. Pool looks crystal clear and the only abnormality I am seeing is a grayish colored film on top of the water inside my skimmers. Also the back side of the skimmers were coated black along the water line. At first I thought the coating on the skimmer wall at the water line was sunscreen from the kids. Now I am wondering if this grayish colored film is what's sticking to the back of the skimmer and is it possibly what is causing the FC loss? Could this be biofilm in my cartridge filter? I've read other posts where Chemgeek talks about biofilms accumulating in sand filters and causing huge loss of FC. I haven't noticed any other accumulations of algae anywhere else in the pool. Just wondering if maybe I should take my filters out and clean them but my water pressure is normal on the filter. Once I get the FC under control, I will see if this film on top of the water disappears. Just curious if anyone here thinks this might be biofilm.