Last year my CYA was 60, after years of trichlor use (or abuse). I opened this season with zero. So did two other gunite IGP owners who live within half a mile from me. None of us had ammonia. I figured we "joined the club" of those afflicted by winter CYA eating bacteria, or another common CYA consuming "stuff" that affects IGPs when they have their winter dormant season.
After opening this season I added CYA to 30, and FC consumption was reasonable for more than a week, but suddenly I saw a drop in FC. I measured CYA, and it dropped to 23. No ammonia is present.
My buddy, who also adjusted his CYA from 0 to 30 just recently, also had a drop to 20!
What happened to my CYA? I also started to add borates. So far added half the calculated amount.
Borates assumed 25 ppm, not verified yet.