I have a TF-100 and have been testing CYA pretty regularly during the swimming season here. Last August/September I was getting CYA readings around 30. This spring when I tested, I was getting a reading of 0...it was as clear as if I was testing tap water. The test is working - I tested a friend's pool and it was reading about where I'd expect it compared to last year. There can't have been more than a 1/4 exchange of water between last fall and this spring (and that's an extremely generous estimate). I've been adding CYA slowly to bring it back up but I'm just wondering what happened to my CYA over winter? Anyone seen this before? I thought CYA was forever...