Question on CYA and CC


Well-known member
May 29, 2017
Worcester, MA
Hey folks:

I'm in my 3rd year with my 40+ year old Gunite Pool and using the TFP Method. Thanks for all the help thus far!

Overall, my water is great and pretty balanced, all things considered.

I've noticed a couple of things when I test recently that I'm hoping you can help me understand. Specifically:

1) My CC always seems to be 0.5 (or higher than 0.0) When I complete the test for FC I always add the 5 drops of the other chemical and the water gets a tinge of pink. I then add one drop of the other chemical and it goes back to clear. I know Pool School says I should slam if CC > 0.5 (which this isn't.) But should I be concerned that it is consistently above 0.0 and < = 0.5?

2) Why does my CYA keep dropping? My pool gets a lot of direct sunlight from about 7 AM until about 3-4 PM. Last year, it always seemed my FC was drifting up and I had to keep dialing down my SWG. My CYA last year was around 70 and I had to adjust it up once during the season. But this year, it seems my FC is hard to keep up (I can do it, but my SWG is dialed in at 80% now). I added CYA about 2-3 weeks ago and it was a lot better. CYA was about 75 (but seems to be some high degree of inaccuracy in the test.) But now 2 weeks later my CYA seems to have fallen back down to 55-60.

What causes CYA to drop? We have had a lot of rain, but I don't think that is the problem.

I"ll add more CYA. I just want to know why it keeps dropping rather rapidly.



Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
CC of 0.5 is normal and shows your chlorine is doing its thing with normal sanitation.

CYA test is accurate to maybe +/-10 ppm. So a swing of 10-20 ppm can be normal testing error. CYA also degrades some and will drop over time.

We always round up so if the dot disappears between 80 and 70 you call it 80. You can’t measure CYA 75 as the markings are logarithmic, not linear.


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
CYA is lost when the pool overflows. So if that has happened when it rains, you lost some there.
Elevated water temperatures can accelerate CYA degradation.