Ok, so I used a friends test reagents and found that I am losing about 6-7 ppm from sundown to sun up the following day. I am still getting a < .5 cc reading also. My water looks crystal clear, but these numbers seem to indicate I am battling something. Is it possible that my cartridge in my filter is harboring some unseen stuff that I havent been able to kill?? I have had my pool at above 20 ppm chlorine for three days straight (shock level) and am still getting huge chlorine losses overnight and during the day. Any insight would be helpful.