Cyanuric acid (CYA), commonly called “stabiliser”, protects chlorine from breaking down under UV rays — without it, an outdoor pool in the sun would lose its chlorine within hours. The problem is that it doesn’t break down or evaporate on its own. Whatever you add (typically along with stabilised tablets) stays in the water until diluted by a water change.
How “chlorine lock” happens
At low to moderate CYA levels (30–50 ppm), stabilisation works exactly as intended. But with every additional dose of stabilised tablets, the CYA level keeps rising — and above roughly 80–100 ppm, cyanuric acid starts to essentially “wrap around” chlorine so much that its disinfecting power drops sharply, even though testers show a normal free chlorine reading. This is called chlorine lock: chlorine is present but practically not working.
How to recognise this situation
- You keep raising chlorine doses, but the water still clouds up or grows algae.
- The free chlorine test shows a normal value, yet something’s clearly off.
- Only stabilised tablets have been added for years, with the water never changed, only topped up.
The only way to be sure is a separate cyanuric acid test (standard pH/chlorine strips don’t measure it).
The fix: dilution, not more chemistry
Cyanuric acid can’t be chemically “broken down” or removed with a product — the only way is a partial or full water change to dilute the concentration back below 50 ppm. For a pool that means draining and refilling with fresh water (a partial change is often enough unless the value is extreme); for a hot tub, this is usually just part of the regular water-change cycle anyway.
Prevention going forward: alternate stabilised tablets with unstabilised chlorine (CYA-free granules or liquid hypochlorite), especially for pools whose water volume stays the same for a long time without being changed.
FAQ
Is high cyanuric acid the same as high chlorine?
No, they’re two separate values measured independently. You can have low chlorine and high CYA at the same time — and that’s actually the worst combination.
Does a hot tub need stabiliser?
Usually not to the same extent as an outdoor pool — a hot tub is typically covered and less exposed to UV. Unstabilised chlorine or bromine is more suitable there.
How often should I test CYA?
At the start of the season, and again whenever chlorine “stops working” despite the correct dose — no need to check it weekly.
Cyanuric acid testers and unstabilised chlorine forms are in the water treatment category. More articles on pool and hot tub chemistry are in the Guides section.
