@PoolDoc, I don't know if you've read through this whole thread, so here's my quick summary from post #59:
mary jo's pool has CYA of about 150ppm, CH of 750, a history of trichlor use, periods of zero measurable chlorine, and low pH. There's no visible algae, and high chlorine demand. The Nature 2 cartridge is in place but hasn't been replaced in the last two years. There is a possibility that some amount of fertilizer made it into the pool. My guesses are that some CYA may have been converted to ammonia causing chlorine demand (no bad smell so probably not); the fertilizer may be causing chlorine demand directly; or that a nascent algae bloom (possibly aided by fertilizer) is causing chlorine demand.

Since I wrote the summary, the pool has been brought and kept at 30ppm FC and has been there for some time. During the shock, the chlorine demand seems to be subsiding but still about 7-8ppm /day with a consistent 3ppm CC.

Thanks for looking in.