Just finishing my first season with a new pool, and had another 'cya disappeared' episode. Spent most of the week rebalancing. This is now the third time I've had to reset cya levels.
Dec 2011 - Pool Filled
Jan 2012 - Set pool to 30-40 CYA
April 2012 - All CYA seemed to have gone, added again till it was brought up to ~50 CYA
Oct 2012 - Noticed that FC readings had been trending down for a 4-5 days, adjusted SWG to generate more, and then over 48hrs - the ORP sensor attached to my intellichem indicated that the pool CYA had disappeared, and sure enough a CYA test showed had fallen to under <20.
Note that the SWG is still reporting normal salt readings, and the borate test strips I have here are still showing borate readings within range. Also, given that I can tell how off the cya is by the ORP...the change would have had to have happened within a few days, and I know my autofill is controlled by a timer which limits max usage to no more than a few hours/day and there is no way that a water leak sufficient to drop CYA that much could have been masked by that limited of an autofill runtime.
What I suspect is that there was another algae outbreak into the pool and it went through the chlorine first and then attacked the CYA...we have kids in the neighborhood over every monday for swimming lessons. I'm assuming one of them brought something into the pool somehow...
Anything more I can do to prevent this type of event other than to be more responsive to sudden increases in chlorine usage? I'm not sure if I should fault myself for not shocking right away....swimming lessons were on Monday, I noticed CC was 1.5 on Tuesday...was concerned but held off on shocking right away (just increased SWG runtime)..but by Thursday morning, it was clear that the CYA had been removed/eaten..