Split by moderator from HERE. jblizzle
Hey I'm having same problem. Scratching my head here in Austin. I had drained 40% of my pool due to really high cya (ignorant puck user pre-TFP). This reduced cya to 40 and all else was good (CH a bit low @ 140) until today (1 week later) when I just measured -40 after only 1.5" of rain! Water is clear. wth is going on? I tested twice thinking that surely I had screwed up. That's a huge drop in a week!
Test results from 6-16-15 after big rain (over an inch)
FC 0 (added 2 bottles bleach, 96oz ea of 8.25%). Now ~0.4
CYA -40 (was 40ppm just 5 days ago)
Water crystal clear - running pump 6 hrs / day with very good circ
Perplexed in Austin