You keep adding more and more chlorine to your pool, the test strip shows decent numbers, yet the water still turns cloudy and algae clings to the walls? Chances are the chlorine itself isn’t to blame — its stabilizer, cyanuric acid (CYA), is. In small amounts it’s essential, because it shields chlorine from sunlight. In excess, though, it effectively switches chlorine off: the chlorine is in the water, it just can’t keep up with disinfecting. This article explains how the stabilizer works, what levels to maintain, and how to climb out of so-called over-stabilization.
What Is Cyanuric Acid, and Why Does Chlorine Need It?
Cyanuric acid (CYA) is a white powder or granulate sold as “chlorine stabilizer.” In water it bonds loosely and reversibly with free chlorine. That bond acts like sunscreen for chlorine: it shields hypochlorous acid molecules from ultraviolet radiation, which would otherwise break them down very quickly.
Without stabilization, the losses are dramatic. Industry sources suggest that direct summer sun can destroy half the free chlorine in unstabilized water within tens of minutes, and up to 90% of the dose can vanish within two to three hours. In other words: dose the pool in the morning, and by noon there’s practically nothing left to disinfect it.
With stabilizer at the right level, chlorine in an outdoor pool stays effective all day and beyond. That’s why CYA is either added on its own at the start of the season, or comes built into so-called stabilized chlorine — dichlor and trichlor tablets. And this is exactly where the problem most guides never mention begins.
What Are the Right CYA Levels?
For an outdoor chlorine pool, it’s recommended to keep cyanuric acid at roughly 30–50 ppm (mg/l). Below 30 ppm, chlorine degrades quickly in sunlight; above 50 ppm the extra protection barely increases while the downsides start to outweigh the benefits.
- Uncovered outdoor pool: 30–50 ppm.
- Saltwater pool (salt chlorination): manufacturers often recommend 60–80 ppm, since chlorine is generated continuously in small amounts and needs longer-lasting protection.
- Indoor pool: CYA is unnecessary — no UV exposure, no need for stabilization.
- Hot tub under a thermal cover: practically no need at all. The water stays covered most of the day, so UV never reaches it. The small amount of CYA introduced by the initial dichlor dose is enough — don’t add stabilizer to a hot tub on purpose.
An important detail: cyanuric acid essentially never breaks down on its own in water. It doesn’t evaporate, doesn’t filter out, and isn’t degraded by sunlight. The only thing that reduces it is dilution — rain overflowing the pool, backwashing the filter, and above all replacing part of the water.
How Do You Dose Stabilizer at the Start of the Season?
Dosing is simple: 1 gram of cyanuric acid per 1 m³ of water raises the level by 1 ppm. For a 30 m³ pool with a target of 40 ppm, you’d need 1.2 kg of stabilizer. Granulate dissolves slowly, sometimes over several days — the tried-and-tested trick is to pour it into an old sock or a skimmer sock and hang it in the skimmer while the pump is running.
If you’re filling the pool for the first time, or after a complete water change, dose the stabilizer as one of your very first steps — chlorine added to unstabilized water in the sun is essentially money thrown away. Re-test the level only after a week; testing sooner will show less than what’s actually in the water, because the granulate dissolves slowly, and tempts you into needlessly overdosing.
Over-Stabilization: When Chlorine “Is There” But Doesn’t Work
This brings us to the core of what’s commonly called “chlorine lock.” If you chlorinate your pool long-term with stabilized chlorine — typically trichlor tablets in a floater — every tablet adds not just chlorine but another dose of cyanuric acid too. The chlorine gets used up, the CYA stays behind, and month after month it climbs: 60, 100, 150 ppm or more.
The more CYA in the water, the larger the share of chlorine bound to it, and the slower that chlorine reacts. Meanwhile the tester still shows “free chlorine within range,” because standard tests also measure chlorine temporarily bound to the stabilizer. The result: water that looks healthy on paper while algae and bacteria multiply happily.
How Much Chlorine Do You Need at a Given CYA Level?
The Trouble Free Pool community (based on calculations by chemist Richard Falk) arrived at a simple rule: minimum free chlorine should be roughly 7.5% of the CYA value, with a target of 10–15% for normal operation. In practice, that means:
- CYA 30 ppm → at least about 2 ppm free chlorine,
- CYA 50 ppm → at least about 4 ppm,
- CYA 100 ppm → at least about 7.5 ppm,
- CYA 150 ppm → over 11 ppm of free chlorine — a level virtually no home pool ever maintains.
And that’s the whole secret behind “ineffective” chlorine: at CYA 120 ppm and 3 ppm chlorine, the tester’s chart says everything’s fine, but in reality there’s so little active chlorine that it can’t protect the water. And because the stabilizer never leaves on its own, dumping in more chlorine will never permanently solve the problem.
How Do You Know You’re Over-Stabilized?
The first and most reliable step is to test your CYA level. Standard test strips include a field for cyanuric acid; a drop-based turbidity test or a photometer is more accurate. If this is your first time testing, check out our guide on how to properly test pool and hot tub water.
Typical signs of over-stabilization:
- CYA above 70–100 ppm (note: many tests only read up to 100 ppm — the real value may be higher; diluting the sample with distilled water gives a more accurate reading).
- Chlorine reads “within range” on the tester, yet you still see algae and green water or slippery walls.
- Shock chlorination only works for a day or two before the problem returns.
- The pool runs on trichlor tablets alone all season, and the water hasn’t been changed in years.
The rough math tells its own story: a trichlor tablet adds about 0.6 ppm of CYA for every 1 ppm of chlorine. A 30 m³ pool burning through a typical 2 ppm of chlorine a day from tablets will “stockpile” over 150 ppm of stabilizer across a five-month season — if nothing dilutes the water.
How Do You Fix Over-Stabilization?
Partial Water Replacement — The Only Reliable Fix
Because CYA doesn’t break down chemically, the only reliable solution is to drain part of the water and top up with fresh water. A simple rule of thumb applies: replacing 30% of the water lowers CYA by roughly 30%. At 120 ppm with a target of 40 ppm, you’d need to replace roughly two-thirds of the volume — you can split that into two steps a few days apart.
There are also “CYA reducer” products on the market based on enzymes or bacteria; based on experience from industry communities, though, their results tend to be unreliable and slow. Treat them as, at most, a supplement — never a replacement for a water change.
Switching to Unstabilized Chlorine
To stop the problem from coming back, stop using stabilized chlorine as your main disinfectant after the water change. For everyday use, choose sodium hypochlorite (liquid chlorine) or calcium hypochlorite (granulate or slow-dissolving tablets) — both deliver chlorine without a single milligram of CYA. That way you keep stabilizer at 30–50 ppm separately, fully under your own control.
Chlorine Types and How Much Stabilizer They Add
| Chlorine type | Form | Adds CYA? | Added per 1 ppm chlorine | Best suited for |
|---|---|---|---|---|
| Dichlor (sodium dichloroisocyanurate) | fast-dissolving granulate | yes | approx. 0.9 ppm CYA | starting dose, hot tubs (short-term), shocking CYA-free water |
| Trichlor (trichloroisocyanuric acid) | slow-dissolving tablets | yes | approx. 0.6 ppm CYA | vacations, short-term dosing; long-term risk of over-stabilization |
| Calcium hypochlorite | granulate, tablets | no | 0 (raises calcium, approx. 0.7 ppm) | routine and shock chlorination in soft water |
| Sodium hypochlorite | liquid chlorine | no | 0 (slightly raises salinity) | everyday chlorination, dosing stations |
| Salt chlorination | salt cell (generator) | no | 0 (chlorine generated from salt) | automated operation; compare in our article on salt water vs. chlorine |
The table makes the key principle clear: stabilized chlorine is a great servant early in the season and a poor master by July. Dichlor and trichlor make sense until CYA reaches the target 30–50 ppm. From that point on, disinfection should be handled by unstabilized chlorine or a salt cell instead.
How Do You Prevent Over-Stabilization?
- Test CYA at least once a month, not just chlorine and pH. Without knowing your CYA level, you don’t actually know how much chlorine you need.
- Keep the 7.5% ratio in mind: as CYA rises, your target free chlorine has to rise proportionally too — or CYA needs to come down.
- Use tablets deliberately: reach for trichlor to bridge a vacation, not as your year-round routine.
- Don’t worry about stabilizer in a hot tub: under a cover there’s no UV exposure, and a regular water change every 3–4 months keeps CYA low on its own.
- Don’t shy away from dilution: every filter backwash and top-up with fresh water knocks CYA down a notch. Think of it as maintenance, not waste.
Frequently Asked Questions (FAQ)
Is cyanuric acid harmful to health?
At the concentrations typical in pools (up to 100 ppm), CYA isn’t acutely dangerous to swimmers. The problem is indirect: a high level weakens disinfection, allowing microorganisms to survive that chlorine would otherwise reliably kill off. That’s why hygiene limits for public pools cap the CYA level.
Does shock chlorination reduce cyanuric acid?
No. A shock dose of chlorine doesn’t break CYA down — the stabilizer rides out the shock unchanged, and once the chlorine is used up, everything’s back to where it was. The only reliable way to bring it down is a partial water change.
How much CYA does a hot tub under a thermal cover need?
Almost none. A covered hot tub isn’t exposed to UV, so there’s nothing for chlorine to be protected from. The small amount of CYA from an initial dichlor dose is harmless — just don’t add stabilizer on purpose, and regular water changes will keep the level naturally low.
Can I use trichlor tablets all season?
For an uncovered pool with frequent fresh-water top-ups, it can work, but you need to keep an eye on CYA. Once it climbs past roughly 50–60 ppm, switch to sodium or calcium hypochlorite, or you’ll end up over-stabilized by the middle of summer.
Why does my tester show good chlorine, yet the water still turns green?
Standard tests measure free chlorine including the portion bound to the stabilizer. At high CYA, the number on your tester looks fine, but the active chlorine is a fraction of what’s actually needed. Test your CYA and compare: if free chlorine is below 7.5% of the CYA value, you’ve found your culprit.
A properly dialed-in stabilizer is the difference between a pool that demands chlorine every single day and one that sails through a sunny week without a fuss. You’ll find cyanuric acid testers, unstabilized chlorine in both liquid and granulate form, and complete pool water chemistry in our water treatment category — plus more practical guides in our Advice section.
