What happened to my CYA


New member
Jun 30, 2020
Encino, CA
Pool Size
Liquid Chlorine
It’s been one year that I have been maintaining my pool.
I consistently had CYA above 90. So I’ve been keeping my FC around 10. Water is crystal clear and I only use liquid chlorine thanks to TFP.
My April reading was again CYA 90-100. However, in the past few months we have been experiencing very hot summer here in So. California. Almost every day or so I have to add chlorine to maintain my high FC. I figured it’s because of the sun exposure.
I did a CYA test after 3 months as I have been doing always and now it reads CYA 20-25. No drain.
I even bought a new CYA reagent thinking my original CYA reagent was old. Same result.
I did add 2 rounds of 18oz. PoolTec last month in hopes of not losing FC as fast but that didn’t work.
So what happened to my CYA? (Which I’m not complaining. Keeping high FC becomes very costly).
Should I be adding stabilizer now.


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
Pool Size
Salt Water Generator
SWG Type
Pentair Intellichlor IC-60
High pool water temperatures will cause the chlorine to oxidize Cyanuric Acid. This tends to show in water temperatures of 90+ degrees. Every 10F increase in temperature results in roughly doubling the rate of degradation.

Chlorine breakdown in sunlight causes CYA degradation by hydroxyl radicals. This can cause a loss from 2 ppm per month to 10 ppm per month depending on the amount of sunlight the pool is exposed to.

In an area with 90+ pool water temperatures and extreme sunlight exposure 10+ ppm of CYA a month can be lost through degradation.

You need to add stabilizer.
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