CYA and CH question


Mar 29, 2021
Chico, CA
My CYA is currently sitting at 45 and my CH at 400. Will these numbers trend up or down with splash out and fill water? My fill water has a CH of 80. I'm just wondering if I should expect these numbers to increase or decrease over time. I would think the CYA would slowly go down, but the CH doesn't evaporate so will it concentrate in the water? Would the low fill water CH have an effect?

As always thanks for all the amazing help on this site.


Gold Supporter
Jun 28, 2017
Fresno, CA
Pool Size
Your thinking is correct. CYA will drop slowly over time but no need to test more frequently than once per month. CH will rise as your pure water evaporates, leaving the calcium behind and you add fill water. While the CH of your tap water is not very high it will accumulate over time. Track your CH monthly as well and keep a close eye on your CSI (calcium saturation index) in Pool Math to see how the rising CH when relative to your current TA, pH, water temp and even salinity changes over time. Also there is no 45ppm CYA you must round up so use 50ppm as your number.


Mar 29, 2021
Chico, CA
Ok, thank you. On the CYA test I lost the dot right between the 40 and the 50 so I was considering it 45ppm. At what point should I be concerned my CH is too high? When it hits the top of the recommended range (650)? On average does it take quite a bit of time to climb? I have been and will continue to keep an eye on the CSI, it is currently -0.07.

Rancho Cost-a-Lotta

Silver Supporter
Apr 10, 2018
Rancho Cucamonga, CA
Pool Size
Salt Water Generator
SWG Type
CircuPool RJ-45
My fill water CH and climate are similar to yours. I see a CH rise of 50 to 100 ppm per year depending on how much water I can replace with rainwater during the off season.

Depending on your water chemistry, you should be able to manage CH levels of 850 +. Bump up your CH numbers in pool math and see how it affects CSI. You can offset the effects of increasing CH by lowering pH and TA.
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