Quote Originally Posted by chem geek View Post
Since you said you checked your filter, I assume you checked other places such as your skimmer and pump basket. You might also check under any removable ladders and perhaps behind light niches. Something is seriously consuming chlorine in your pool, and losing it overnight means it's not the UV in sunlight.

I also assume you test your chlorine level within an hour after you've added it just to make sure your chlorine isn't weaker than you think. So I too am at a loss. This is very unusual. Even if it were some conversion of CYA to ammonia and/or partially oxidized CYA, you should see your chlorine demand dropping and you would see at least some CC.
Yes, I do check chlorine levels after adding. For example, last night at 9 PM I added 8 gallons of 6% bleach and raised my FC level from 19.5 ppm to 42-43 ppm which it tested at 11 PM. This morning at 7 AM, my FC level was 37 ppm, for a loss of 4-5 overnight. I had a similar loss the night before of 5-6 ppm. No cc reading this morning, either. I will check behind my light housing today. Pump basket is clean. Under ladder looks clean, too.

I am at a loss here and it is getting very expensiive!!

The reason I took it up to high 30's for FC is that keeping it at the shock level recommended by the BBB method wasn't working. The chart recommends 20 ppm for shock level for me.