Cyanuric acid in a pool: what it does and why it only goes up
Cyanuric acid — stabiliser — is the most misunderstood number in pool water. It protects your chlorine from sunlight, which is genuinely useful. It also accumulates relentlessly, and above a certain point it actively makes your chlorine less effective. Nothing you do in normal maintenance brings it back down.
What cyanuric acid actually does
Sunlight destroys free chlorine. In an outdoor pool, unstabilised chlorine can lose most of its strength in a couple of hours of direct sun. Cyanuric acid binds to the chlorine and slows that destruction dramatically, so the sanitiser you added is still there in the afternoon.
That is why outdoor pools use it and indoor pools generally do not — there is no sunlight to protect against, so the downside is all you would be buying.
Why it only goes up
Cyanuric acid is not removed by filtration, by backwashing, or by any chemical you would normally add. In practice it behaves as a one-way ratchet: every time you add stabiliser, or use a stabilised chlorine product, the level steps up and stays there.
The only practical way back down is to remove water and replace it with fresh water. That is why it pays to add less than you think the first time — overshooting means draining part of the pool.
Why too much is worse than too little
Cyanuric acid slows chlorine's reaction rate against everything, not just sunlight. At higher levels, the same free chlorine reading is doing less work on contaminants and algae. This is why a pool can report perfectly respectable chlorine and still go green.
It is the single most common hidden cause of recurring algae. If your chlorine looks fine and the algae keeps coming back, check this number before adding anything else. Our mustard algae and black algae pages both come back to it.
The range you are aiming for
| Pool type | Commonly cited CYA range | Why |
|---|---|---|
| Outdoor, chlorine-fed | 30–50 ppm | Enough to protect chlorine from sun, not so much that it slows it down |
| Salt water (chlorinator) | 60–80 ppm | Salt cells run at a lower, steadier output, so a little more protection is usual |
| Indoor pool | Generally not needed | No sunlight to protect against, and all of the downside |
These ranges are read straight from our source register, which also states what still needs credentialed review. They are guidance ranges, not our own recommendations.
If you are adding stabiliser, work out the amount from your current reading and your target — and if you are between two amounts, add the smaller one.
How to lower cyanuric acid
- Confirm the reading with a reliable test. Test strips are the least accurate for CYA; a turbidity test or a lab reading is better.
- Work out what fraction of the water you need to replace, based on how far above range you are.
- Drain part of the pool and refill with fresh water. Partial drain and refill, repeated if needed, is the standard method.
- Re-test after the water has mixed properly — not immediately.
- Fix the source. If you use stabilised chlorine products routinely, that is where it is coming from.
Where it comes from without you noticing
- Dichlor and trichlor — stabilised chlorine products — add CYA every time you use them. Over a season this is a significant accumulation.
- Cyanuric acid sold as 'stabiliser' or 'conditioner', added deliberately.
- Some shock products contain stabiliser; check the label rather than assuming.
- Topping up after backwashing does not add CYA, but it does dilute it slightly — which is why backwashing too often slowly lowers the number.
If you use trichlor tablets in a feeder all season, assume CYA is climbing whether or not you have added stabiliser. Test it monthly.
Cyanuric acid questions
Can you filter cyanuric acid out of a pool?
No. It is dissolved, not suspended, so no filter removes it and backwashing does not touch it. Replacing water is the only practical method.
Does shock lower cyanuric acid?
Not meaningfully at the levels you would use. Some specialised treatments claim to, but they are not a substitute for replacing water.
What happens if CYA is too low?
Your chlorine is destroyed by sunlight quickly, so you burn through sanitiser and struggle to hold a residual. Very low CYA in an outdoor pool is a real problem — just a much easier one to fix than too much.
Do I need cyanuric acid in an indoor pool?
Generally no. There is no sunlight for it to protect against, so you get the downsides only.
Is high CYA dangerous?
It is not acutely toxic, but it makes your sanitiser ineffective, and ineffective sanitation is the actual health concern. It also means you need to drain water to fix it.
Related
- CYA / stabiliser calculator
- Mustard algae in pool — why high CYA makes it worse
- Black algae in pool
- How to backwash a pool filter
- Pool salt calculator
Review status: Chlorine and stabiliser. Last reviewed 2026-09-13. See the source and review status of every formula →