Without stabilizer, North Texas sun would burn through the chlorine in your pool faster than you could replace it. We've seen pools at zero ppm CYA lose 90% of their free chlorine in a single afternoon. Stabilizer — cyanuric acid — is what makes outdoor chlorine pools possible at all in this climate.
But like everything else in pool chemistry, more is not better. Too much CYA is one of the most common reasons we get called out to a pool that "won't hold chlorine." So here's the whole story.
What CYA actually does
Cyanuric acid binds loosely to chlorine. The bound chlorine is protected from UV degradation, but it's also temporarily inactive — it can't sanitize while it's bound. As chlorine in the water gets used up, more is released from the CYA bond. Net effect: a slow, steady supply of available chlorine instead of a flash that disappears at noon.
A useful (imperfect) analogy: CYA is sunscreen for chlorine. A little protects it. Too much smothers it.
Target CYA range
Standard chlorine pools: 30–50 ppm is the sweet spot for Texas sun.
If you can get away with 30, do. If your pool is in full sun and you struggle to hold FC, 40–50.
Salt water pools: many SWG manufacturers recommend higher — 60–80 ppm — to protect chlorine generated at lower output.
Indoor pools: 0–10 ppm. CYA isn't needed indoors and creates problems if it accumulates.
Why CYA almost always drifts up
CYA does not break down on its own at any meaningful rate. It only leaves your pool through:
Backwashing.
Splashout.
Pool drain-down or rain overflow.
Meanwhile, every time you use a stabilized chlorine product (trichlor tablets or dichlor shock), you add CYA. A typical trichlor tablet is about 50% CYA by weight. Three tablets a week, all summer, in a 20,000-gallon pool — without any backwashing or water changes — and you'll add 30+ ppm of CYA in a season.
That's how a pool that started at 40 ppm in May ends up at 90 ppm in September. Most homeowners don't notice because the chlorine reading still looks normal for that day. But the pool is becoming progressively harder to sanitize.
How to tell if your CYA is too high
Free chlorine reads "fine" but combined chlorine creeps up.
You're adding more chlorine than you used to to get the same FC reading.
Algae shows up despite normal-looking chlorine.
Shocking the pool doesn't seem to do anything.
Your pool stays cloudy or slightly off-color.
If any of those sound familiar, test your CYA. The test is a turbidity test — you mix water with a reagent and read it on a calibrated tube. It's slightly squishy compared to other pool tests, but it doesn't need to be exact. If your reading is "somewhere between 70 and 90," that's already a problem.
Lowering CYA — the bad news
There's only one practical way to lower CYA: replace water. Drain a portion of your pool and refill with fresh water.
25% drain-and-refill cuts CYA by ~25%.
50% drain-and-refill cuts CYA by ~50%.
And so on.
This is why we tell customers in Ellis County: don't let CYA get away from you. Once it's at 100, you're looking at a meaningful water change to fix it. In a drought year (which we get more of than we used to in Texas), that's not nothing.
There are commercial CYA-removing products — they're enzymes that break down cyanurate. They work, but slowly, expensively, and not as reliably as a partial drain.
Raising CYA — the easy direction
If your CYA is below 30 and you want to raise it: dissolve granular cyanuric acid in a sock, hang it in front of a return jet, and let the pump run for 24–48 hours. Don't dump it dry — it takes a week to dissolve and forms gummy lumps in the bottom of the pool.
To raise CYA by 10 ppm in 10,000 gallons, you need about 13 oz of granular stabilizer. The Cyanuric Acid Calculator does the math.
How to keep CYA in range long-term
Use liquid chlorine or cal-hypo as your primary sanitizer instead of trichlor tablets. Both are unstabilized.
If you use tablets at all, use them as a "vacation backup," not a full-time sanitizer.
Backwash on schedule — backwashing removes water, which removes CYA.
Test CYA at least quarterly.
When to call us
If your CYA is over 80, we'd rather come look at it than tell you to dose blind. A partial drain of an Ellis County pool requires knowing your liner type, your wall structure, your hydrostatic relief situation, and the local water table. We'd rather not have you popping a vinyl liner trying to fix a chemistry problem.
Run the Cyanuric Acid Calculator or call Thomas directly at (214) 399-7347.
