Cyanuric acid (CYA) — also called pool stabilizer or conditioner — is one of the most important and most misunderstood chemicals in pool service. Myths about CYA are responsible for pools that maintain inadequate sanitation, pools that waste enormous amounts of chlorine, and service techs who can't explain to customers why their pool keeps struggling. Getting CYA right is foundational. Here's the full picture, with the myths cleared away.
CYA protects chlorine from UV photolysis — it forms a weak chemical bond with hypochlorous acid (HOCl), the active sanitizing form of chlorine. Without CYA in an outdoor pool, sunlight destroys 90% of free chlorine within 2 hours. With CYA at 30–50 ppm, chlorine half-life in direct sunlight extends to 6+ hours.
The myth: adding more CYA provides more protection. The reality: there's a ceiling. Above 50–60 ppm, the additional protective benefit is marginal. And CYA doesn't just protect chlorine from UV — it also slows chlorine's ability to react with pathogens and organic compounds. The same bond that shields chlorine from sunlight also reduces its sanitizing potency.
The FC/CYA relationship defines the minimum effective free chlorine:
| CYA (ppm) | Minimum FC for Sanitation | Target FC (Algae Prevention) |
|---|---|---|
| 0 | 0.5 ppm | 1 ppm |
| 30 | 1.5 ppm | 2 ppm |
| 50 | 2.5 ppm | 4 ppm |
| 80 | 4 ppm | 6 ppm |
| 100 | 5 ppm | 7.5 ppm |
A pool with 100 ppm CYA and 3 ppm FC is less protected than a pool with 40 ppm CYA and 3 ppm FC. The numbers on the test kit don't tell the story — the ratio does.
This is completely false and accounts for enormous wasted chemical spending on stabilizer additions. CYA is one of the most chemically stable compounds in pool water. It does not degrade with sunlight, does not evaporate, and is not consumed by chlorine or pool chemistry reactions. The only way CYA leaves a pool is through water replacement (splash-out, backwash, rain dilution, drain and refill).
In pools that use trichlor tablets as their primary sanitizer, CYA accumulates continuously. Trichlor is approximately 54% CYA by weight — every tablet addition increases stabilizer levels. Without periodic dilution or a switch to non-stabilized chlorine sources, CYA in tablet-fed pools can reach 150–200 ppm over multiple seasons.
This is the myth with the most serious safety implications. When CYA is very high (above 100 ppm), standard FC test results can be deeply misleading. A pool showing 3 ppm FC at 150 ppm CYA has almost no effective sanitizing power. The "chlorine lock" effect — where virtually all free chlorine is bound to CYA and unavailable to kill pathogens — can make a pool look chemically normal while being dangerously undersanitized.
The CDC has documented disease outbreaks in pools with extremely high CYA levels where standard chlorine concentrations were ineffective against Cryptosporidium and other pathogens. These were not negligence cases — the pools tested as "in range" at the point of chlorine measurement. Managing CYA within recommended ranges is a health and safety matter.
CYA does not kill algae. Adding stabilizer to a green pool does not help — it potentially makes the situation worse by reducing the effectiveness of the chlorine treatments you're adding. A green pool needs aggressive FC treatment (shock), not stabilizer. If CYA is already high when the pool goes green, the shock treatment needs to be calibrated to the elevated CYA level — often requiring 10+ ppm FC to break through.
Some old-school service guidance cited 60–80 ppm as the target range for CYA. Current PHTA recommendations and the Pool Chemistry Training Institute (PCTI) position this lower: 30–50 ppm for most outdoor residential pools. The lower target preserves more active chlorine effectiveness while still providing meaningful UV protection.
For saltwater pools, the PHTA recommendation is 60–80 ppm — saltwater systems produce chlorine at lower rates, and the additional CYA protection is useful to reduce UV consumption between generation cycles.
Accurate CYA testing requires a turbidity comparison method — the Taylor reagent tube test is the standard. Test strips for CYA are notoriously unreliable; they should not be used for CYA measurement on accounts where the number matters. The LaMotte ColorQ Pro 7 does not include CYA testing — you need a turbidity tube test for accurate results.
CYA management protocol for service accounts: test quarterly at minimum. If CYA exceeds 80 ppm, recommend partial drain and refill (typically 30–50% of pool volume). Document the recommendation in writing. If the customer declines, document that as well.
Use PoolLens to calculate your FC targets based on actual CYA — because the right free chlorine number is impossible to determine without knowing the stabilizer level. The FC/CYA ratio is the most important calculation in residential pool chemistry.
PoolLens calculates your target FC based on your CYA level — accurate chemistry, free and offline.
Open PoolLens Free →Cyanuric acid (CYA, stabilizer, conditioner) forms a weak bond with free chlorine in pool water, protecting chlorine from UV photolysis. Without CYA, sunlight destroys 90% of pool chlorine within 2 hours. With CYA at 30–50 ppm, chlorine half-life in sunlight extends to 6+ hours.
PHTA recommends 30–50 ppm for outdoor residential pools. Saltwater pools: 60–80 ppm. Commercial pools: 30–40 ppm. Above 80 ppm, CYA significantly reduces chlorine's sanitizing power, requiring higher FC to maintain equivalent sanitation.
At very high levels (above 100 ppm), CYA significantly compromises chlorine effectiveness against pathogens — particularly Cryptosporidium. The CDC has documented outbreaks in pools with extremely high CYA where standard chlorine levels were ineffective. Maintaining CYA in recommended ranges is a health and safety matter.
The only practical way to lower CYA is dilution — partial drain and refill. There is no chemical treatment that reliably removes CYA from pool water. CYA does not degrade through normal pool chemistry or sunlight. If CYA exceeds 100 ppm, a 30–50% drain and refill is the standard protocol.
Yes — every trichlor tablet adds CYA along with chlorine. Trichlor is approximately 54% CYA by weight. A pool using trichlor tablets as its primary sanitizer will accumulate CYA continuously over the season without periodic dilution or switching to non-stabilized chlorine.