It happened to me! In two days of light usage our CYA went from 80 ppm with crystal clear water to zero with severely milky water. No green algae, no apparent algae at all. We've been trying for 10 days to figure out what happened. Our local pool guy really didn't have a clue. He recommended we use a non chlorine shock every other day until CYA came back up....I did that 5 times with absolutely no change.
Then I found this forum and chem geek's description of his problem. Went to the big box, got an ammonia test kit, and sure enough, 2 ppm ammonia.... So now we're superchlorinating until it will hold. Took it up to 35 ppm (based on amount required for superchlorination with 3.5 CC) and it went to zero FC in 30 min. We did get that "pool smell" - chloramines! So I take that to mean that the ammonia is reacting with liquid chlorine and eventually we will be burning it off with the chlorination. So we'll do what we can tonight by repeatedly adding shock levels of chlorine and watching it come down. Water is clear, all other chemistry seems in line - just no FC and apparently no remaining CYA.
Complicating factor is we are leaving for four days tomorrow. Should we keep the chlorinator running full steam while we are gone? Any other recommendations? When should I put some CYA in again?
I thank chem geek for the blow by blow!! It gives us an idea of what to expect. (Looks like we have "the worst case where there (...is...) ammonia, then the rule you can use is that it takes up to 2.5x the CYA drop as FC to clear it. If it were a 40 ppm drop, then that would be 100 ppm FC. " (chem geek, May 20, this year) Since we went from 80 ppm CYA to zero, we may be looking at 200 ppm FC to get rid of ammonia....in a 30K gallon pool...ouch.)