I added 3lbs of CYA granules to my pool this spring to bring it up to our target of 70ppm, confirmed by testing. I just measured again today because chlorine demand was high and it has dropped to between 0 and 10. My understanding is that the only way to lower CYA is to empty and refill with fresh water. I've been backwashing and filling the pool the usual amount so there's no way I've cycled more than 80% of the water these past few months.
When I first measured I thought my reagent must have gone bad, but the pool store measured the same.
Is there anything else that could account for the disappearance of CYA?