Head scratcher here.
Beginning of the season I added 5 lbs of stabilizer to bring my CYA into range 60-80. So a few weeks ago I needed to increase my pump run time and SWCG output to maintain proper FC levels. (Bells should have been going off but I chalked it up to burn off due to longer days.) WRONG.
We went on vacation for a week and came back to a clear pool but with some algae on the walls, nothing a brushing couldn't take care of. Measured my FC level it was only 2. (I normally run about 6 or so.)
Thinking I haven't run a full set of test in a while, I did, and lo and behold I'm not reading any CYA. CH has gone down, so has salt and borates. Hmm. The plot thickens.
I haven't been adding any more water than normal, just replacement for typical evaporation here in central Florida, not getting the normal afternoon showers yet.
So, I've added 7 lbs of CYA to bring it back into normal range. Added some salt and borates. All is well.
Also brushed the pool well unfortunately Black algae has also taken hold, but its dying out, stainless steel bristle brushing sees to that and a slightly elevated FC level!
Rereading this post, it almost sounds like I have a leak, more splash out, or the fact that I've been lax on my testing. New twins added to the family will take up more time.
Hmmm. Guess I'll keep an eye on it. Maybe a bucket test is called for. I'll update. Chime in if you have ideas or suggestions.