Alright. Two nights in a row I've added enough chlorine to bring the pool up to the shock region on the BGC, and both nights the pool tested much, much lower 2 hours later. Tonight I started with .8ppm FC and added 3.5 gallons of 6%. I now have 2.0ppm. Lat night I added about 3lbs of 73% cal-hypo and 2 hours later it was around 2.0ppm. So two different Cl mediums show some wicked chlorine demand? To recap, the thread started with my missing 2.5 lbs of CYA. Does this sound close to the chlorine demand from ammonia? My pool went from clear green to clear blue last night, but I have a feeling that my adding a lot of the cal-hypo to the skimmer last night allowed my sand filter to finally hang on to whatever was in the water. What's going on?