Cyanuric acid (stabilizer)
Last reviewed August 11, 2026
Bringing it down
| Method | When to use it | Watch out |
|---|---|---|
| Partial drain and refill | The normal approach | Not on vinyl. A drained liner shrinks and may not seat again. |
| Reverse osmosis on site | Drought and hard-water markets | Costs more. Worth it where water is scarce or the fill water is bad. |
| Dilution over time | Small overshoot only | Slow. Relies on splash-out and backwash replacing water. |
Raising it
About 1.3 lb of granular stabilizer raises CYA 10 ppm in 15,000 gallons. Put it in a sock in the skimmer. It dissolves slowly and can take a week to show up on a test. Don't retest the next day and dose again. That's how pools end up at 120.
Cyanuric acid is sunscreen for chlorine. Without it, direct sun burns off most of your free chlorine in a few hours. With too much of it, chlorine is held so tightly that the pool is short on sanitizer at readings that look fine on the kit.
This is the reading that most often makes a pool look healthy when it isn't. The whole trade-off is in the FC–CYA ratio guide.
It only goes up
Stabilizer doesn't break down and doesn't evaporate. It leaves with water and nothing else. So it builds up. Slowly if you dose liquid chlorine, fast if the pool runs trichlor tabs, because every tab adds stabilizer along with the chlorine.
A pool on a tab feeder that hasn't been drained in two seasons will be over 100. That's the usual story behind a "chlorine won't hold" call, and shocking it doesn't help.
How often to test
Monthly is right for most routes. Stabilizer doesn't move in seven days unless something happened. Testing it weekly mostly gives you last week's number again, which is worse than not testing, because it looks like real data.
Test it sooner if the pool was drained or refilled, after heavy rain, when chlorine won't hold, or the first time you service a pool you don't know.
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