UPDATE: Last nite was the first nite my FC held from sundown to sunrise. I pulled my light niche and found a whole lot of crud back there. Very disgusting. So I scrubbed it all out and left the light out for two days so that shocked pool water could circulate in there. Maybe that was the problem with my huge chlorine losses. To re-cap, I had my pool at shock level (above 20 ppm) for at least 12-13 days now. It took that long to defeat whatever was in my pool. Hopefully I am done now. I will keep it up above shoock for at least one more day to be sure.