I am having the same issue. I've read on some other sites that ammonia can cause a fc reading of zero. I take my water samples to a local pool store that does a Bioguard test and I am about $700 deep in chemicals so far. I've put in 30lbs of Lithium hypochlorite and I am currently doing 3lbs of Power Chlor every 2 hours for 14 hours (22lbs) trying to get a fc reading of some sort. I think I may visit Pet Smart and get an ammonia test kit to see if I am dealing with this. The pool stores like to call this a chlorine lock or demand, but it's really just a money pit!
26,500 GAL in ground vinyl pool
Mod note: Split from this thread: Added shock chlorine but sill, FC = 0 ?