For the past few months - read: chilly - my pool has been losing about 1 ppm FC/day, not counting the losses when I dilute with rainwater. I was getting .5 CC off and on when testing FC, never any higher. The rain has stopped for the time being. Pool is all clean and balanced, but I noticed, down inside the main drain, what could be algae. It could also be real fine silt from the rain gutter fill water. All I know is that it looks a dark olive green. The water's too cold to swim down and get a closeup look, and there's no way to get in there with a brush, so I just don't know. Not wanting to risk it, I decided to shock. 16 should be high enough, but I had two full gallons of 12.5%, so I dumped it all in and started at 20.
FC loss was 3 ppm in 24 hours. I topped it back up with some fresh bleach. Again, 3 ppm loss. No sign of CC. The pool is in the sun pretty much all day. I haven't done the overnight test, yet.
What I'm wondering is: does one lose more FC daily as the level increases? You know, like how you lose air pressure faster the higher the starting pressure is? It would make sense to me, as I understand that bleach degrades faster the higher the concentration. Is it one of those logarithmic things?