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I wonder how you can lose at much as 15 ppm CYA. Every year I lose nearly all the CYA over the winter, and I've never been able to figure out why. I just opened last week and the CYA was 10. But i just go to Sam's Club, buy a 50lb bucket of dichlor, and just use that to chlorinate until the CYA is up to 50. But i still wonder what happens to the CYA (and the alkalinity) over the winter.