LifeTime Supporter
Apr 7, 2007
Cary, IL
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.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
If you've got unusually high chlorine demand (but not so high as to seem like ammonia or something like that) and the pool is clear and you've brushed pool surfaces, then it would be a good idea to examine other areas including the pool skimmer, pump basket, behind light niches, under removable stairs, and the filter itself (most especially the filter since that has the largest surface area). You might have a lot of organic buildup in the filters that is reacting with chlorine, but that's just a guess at this point. It doesn't have to be biofilm and could be pollen or other organic material.