When I opened my pool this spring, the CYA was 60 ppm. I checked the level again a few weeks ago and, as expected, it was still 60 ppm.
I went away on travel and vacation from July 14 to 21. When I was away, I set up a Hayward chlorinator with four pucks and had a neighbor monitor the water level. I kept a solar cover on to minimize water and chlorine loss. It was very hot while I was away. The neighbor needed to add water at least two times. The chlorine demand was much higher than I expected and the chlorinator could not keep up.
When I returned, the pool was very cloudy, not green but gray and the chlorine was near zero. I used liquid chlorine to bring the chlorine to shock level for 60 ppm CYA and the pool cleared in about three days. I've been using liquid chlorine now that I'm in town and noticed that the chlorine demand is much higher than before I went away. I also noticed that the demand was twice as great on sunny days.
So, I just remeasured the CYA level. It was zero. Measured again using a new reagent bottle. Still zero. Brought a sample to the local pool store and they measured zero as well. I'm convinced it really is zero.
I'm now adding two pounds to CYA, which should bring the CYA level to around 20 ppm. If this occurs, I'll add another two pounds which should bring me to 40 ppm.
I believe that there was a bloom of anaerobic bacteria while I was away. This is why the pool was cloudy when I returned. The low chlorine level and solar cover probably is what allowed the bacteria to grow. The bacteria consumed the CYA leaving me with zero.
I'm now a believer in CYA-consuming bacteria. Through I never would have thought the level could go from 60 to 0 ppm in nine days. Consider this another data point in the debate.