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Cyanuric acid is the least visible chemical in a pool and the one most likely to be quietly wrong. It is sold as stabiliser, as conditioner, and invisibly inside stabilised chlorine tablets and granules, which is how people end up with far more of it than they intended.
Key figures
- 1 ppm CDC minimum pool free chlorine without stabilizer
- 2 ppm CDC minimum pool free chlorine with stabilizer
What it does
An outdoor pool loses chlorine to ultraviolet light fast. Cyanuric acid binds loosely to chlorine, shielding it from sunlight and releasing it as the free chlorine is consumed. Without it, an outdoor pool in summer can lose most of its chlorine in a few hours.
That is a real benefit, and it comes with a cost: bound chlorine is not working chlorine. The more stabiliser in the water, the higher the free chlorine reading has to be to achieve the same disinfection.
The CDC reflects this in its targets. At least 1 ppm free chlorine in a pool without cyanuric acid, and at least 2 ppm where cyanuric acid is used.
Why hot tubs are different
The CDC is explicit: do not use cyanuric acid in hot tubs.
Two reasons. A hot tub is usually covered, so there is little ultraviolet light to protect against, and hot water with a heavy bather load needs chlorine that acts immediately rather than chlorine held in reserve.
Stabilised chlorine tablets and granules, the trichlor and dichlor products, add cyanuric acid every time you use them. Use them all season and the stabiliser level climbs steadily while nobody is measuring it.
The number goes one way on its own
Cyanuric acid does not evaporate, does not break down meaningfully in sunlight, and is not consumed by sanitising. It leaves with water: splash-out, backwashing, and draining.
So the only way to reduce a high level is dilution: drain part of the pool and refill. That is water, time and money, which is why the sensible approach is to watch the number rather than to fix it later.
Keeping it under control
- Test it monthly as part of the full test, not once a season.
- Know what your chlorine product contains. Liquid chlorine and cal-hypo add none; trichlor and dichlor add stabiliser every dose.
- Alternate products. Many owners use stabilised tablets early in the season and unstabilised chlorine once the level is established.
- Raise free chlorine in step with stabiliser. A stabilised pool needs a higher reading, following the CDC’s at-least-2-ppm guidance as the floor.
- Dilute when it gets high, and time it with a top-up you were going to do anyway.
The symptom that sends people down the wrong path
Water that will not stay clear despite good chlorine readings is a classic high-stabiliser picture: the test says there is chlorine, and the chlorine is not doing much. Adding more chlorine without checking cyanuric acid is how people end up with an expensive month and a green pool.
Test the stabiliser before shocking a pool repeatedly.
Related reading: The Germ Chlorine Does Not Kill Quickly and Opening the Pool Without a Week of Green Water.
Questions people ask
What does cyanuric acid do in a pool?
It protects chlorine from being destroyed by sunlight in an outdoor pool, at the cost of making the chlorine act more slowly. CDC recommends at least 2 ppm free chlorine where it is used.
Should I use stabiliser in a hot tub?
No. The CDC specifically advises against cyanuric acid in hot tubs, where there is little sunlight to protect against and fast-acting chlorine matters.
How do I lower cyanuric acid?
By dilution: drain part of the pool and refill. It does not evaporate or break down in normal use.
Why is my pool cloudy even with chlorine in it?
One common cause is a high stabiliser level, which leaves the chlorine measurable but slow. Test cyanuric acid before adding more chlorine.
Where these figures come from
- Home Pool and Hot Tub Water Treatment and Testing Centers for Disease Control and Prevention · cdc.gov
- Healthy Swimming Centers for Disease Control and Prevention · cdc.gov
Figures last checked September 2026.
