It's been a bit since I've been around... A testament to the fact that my pool has been trouble free this year. That is, except for one thing...
When I opened my pool May 1, I did a full battery of tests. Everything was normal, except 0.0 FC and 0.0 CC. CYA was 40 ppm.
I SLAM'ed the pool, that took about 2 weeks. Ran another full battery of tests post-SLAM, and everything was still fine. CYA at 40 ppm.
I noticed a couple of weeks after my SLAM, though, that I was beginning to see more FC loss from week to week. I started out losing my usual 2.0 ppm FC daily (dosing for 6.0, never actually re-checked, probably ended up slightly higher, though). A couple weeks later, I was losing 2.5 ppm FC daily. A couple weeks after that, I was losing 3.0 ppm FC daily. When I got to the point where I was losing 4.0 ppm FC daily (2 weeks ago), I re-tested CYA.
CYA showed less than 20 ppm. I filled the tube to the top and could still see the dot. It was a bit cloudy, but I could see it plainly. Pool Math called for 67 oz. (weight) of CYA. I bought a 5.5-lb. container and added approximately half (trying to be conservative). FC loss dropped from 4.0 ppm daily to 2.5 ppm daily within 48 hours of adding CYA. I have not re-tested yet (out of reagent...), but now my FC loss has gone back up to 3.0 ppm daily.
I should note that I have not replaced any water and we have not had enough rain to overflow the pool. I have backwashed my sand filter once so far this summer, and that was a week after I added the 2.75 lbs. of CYA. I am adding about 1-1.5" of water per week due to evaporation.
Should I add the rest of the CYA I have on hand? Should I be concerned about this CYA loss? Last year I added CYA up to 40 ppm early in the summer and never saw a decrease. What's different this year?