CYA, CH down. What happened?

Sep 13, 2016
Tucson, AZ
I've experienced an interesting anomaly over the past few months. Back in April I did the complete measurement suite and found that my CH had plummeted to near baseline, and CYA had plummeted to <30 compared to the previous measurements in February. I immediately suspected a leak to the pool or plumbing and so shut off the auto-filler to begin diagnosing the problem (using the bucket test to measure evaporation).

No discernible leak.

Has anyone seen this before? Any speculation on what might have happened?

TIA for any and all ideas.


TFP Expert
LifeTime Supporter
Jan 6, 2010
San Dimas, CA (LA County)
Speculation, yes.

CYA will break down a few PPM per month naturally. Faster if certain bacteria get in the pool and chlorine levels aren't high enough to kill it and it starts eating it.

If the water isn't leaking out and being replenished, then the CH is still there -- most likely as scale on the walls. Do you see any greyish- tan patches? Feel any roughness?
Sep 13, 2016
Tucson, AZ
Update. Turned out to be a broken autofill valve. I'd noticed the water levels were high for a few months but thought it was due to rain. The water was leaking out at the top of the tile (between it and the apron). Replaced the autofill valve and everything is back to normal.