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Cyanuric acid — pool stabiliser — bonds loosely to chlorine and shields it from sunlight, so an outdoor pool holds its residual instead of losing it by mid-afternoon. But the bond works both ways: chlorine held by cyanuric acid is also slower to kill. At sensible levels, roughly 30–50 mg/L, the trade is worth it. The problem is accumulation: every trichlor tablet adds stabiliser that nothing removes, so tablet-fed pools climb steadily. At high levels the pool shows a healthy chlorine reading while disinfection crawls — the condition nicknamed "chlorine lock". The dependable fix is dilution: drain part of the pool and refill. The CDC's Model Aquatic Health Code caps cyanuric acid at 100 mg/L.
Sunlight is chlorine's biggest enemy in an outdoor pool. The ultraviolet part of daylight breaks free chlorine down quickly — quickly enough that an unstabilised outdoor pool can lose most of its residual over a few sunny hours. You dose in the morning; by afternoon the protection is largely gone, exactly when the bathers arrive.
Cyanuric acid solves this with a simple mechanism. It bonds loosely and reversibly with chlorine in the water. Chlorine that is bonded to cyanuric acid is shielded from UV — it survives the sun. Because the bond is reversible, the bonded chlorine is released back as the unbonded portion gets used up. Think of it as a reservoir: a small amount of chlorine free and working, a larger amount held in reserve, and an equilibrium constantly topping up the working share.
That is genuinely useful, and it is why virtually every outdoor pool runs some stabiliser. The catch is in the word loosely. The reservoir is not free.
Chlorine held by cyanuric acid is protected from sunlight — and also largely unavailable for killing germs until it is released. So as the cyanuric acid level rises, a growing share of your measured free chlorine is sitting in the reservoir rather than working, and the concentration of the active killing form falls.
Here is the part that misleads people: your test kit cannot see the difference. Standard free chlorine tests measure the stabilised chlorine along with the active chlorine. A pool at 80 mg/L of cyanuric acid and a pool at 30 mg/L can both read "2.0 free chlorine" — and the first pool is disinfecting at a fraction of the rate of the second. The reading is honest; the interpretation is not.
This is why modern codes pair the two numbers. The CDC's Model Aquatic Health Code requires a minimum of 2 mg/L free chlorine where cyanuric acid is used (against 1 mg/L without), caps cyanuric acid at 100 mg/L, and — since the 2023 edition — treats a cyanuric acid to free chlorine ratio above 45:1 as grounds to close a public pool until it is corrected. The WHO's recreational water guidelines take the same view of 100 mg/L as the level not to exceed.
If stabiliser were dosed once, on purpose, this would be easy to manage. The problem is that most pools dose it continuously without meaning to, because it arrives bundled inside trichlor chlorine tablets.
Trichlor is chlorine chemically bonded to cyanuric acid. When a tablet dissolves, the chlorine part gets consumed doing its job — killing, oxidising, gassing off. The cyanuric acid part does none of those things. It is not used up by disinfection, it does not evaporate, and it passes straight through sand and cartridge filters. The only ways it leaves the pool are splash-out, backwash water, and deliberate draining.
The arithmetic is fixed by the chemistry: every 1 mg/L of chlorine dosed from trichlor adds roughly 0.6 mg/L of cyanuric acid. Dose 2 mg/L of tablet chlorine a day — an unremarkable figure for a warm, busy pool — and you are adding on the order of 1 mg/L of stabiliser every day. Losses from splash-out and backwash claw some of that back, but in a pool with little water replacement the direction is one way. A pool that started the season at a sensible 30 mg/L can be far beyond 100 mg/L within months, with nobody having added "stabiliser" even once.
The pattern to recognise: a tablet-fed pool that used to behave, now fights algae and cloudiness despite normal chlorine readings, and responds to shock dosing for shorter and shorter periods. Before blaming the chlorine supplier, test the cyanuric acid.
The pool trade calls the end state chlorine lock: the chlorine is "locked" and the pool will not come right no matter how much you add. The name is memorable and slightly wrong, in a way that matters for fixing it.
There is no permanent lock. High cyanuric acid does not destroy chlorine or bind it irreversibly — it shifts the equilibrium so far towards the reserved form that the active concentration becomes too small to keep up with the pool's demands. Disinfection does not stop; it slows until algae and bacteria reproduce faster than the chlorine kills them. From the deck, that looks like dead chlorine.
Two practical consequences follow from the real mechanism:
Because nothing consumes cyanuric acid, the practical remedy is to physically take water out and put fresh water in. The sums are as simple as they look:
| You replace | Cyanuric acid becomes | Example: starting at 120 mg/L |
|---|---|---|
| One quarter of the water | About 75% of the starting level | ≈ 90 mg/L |
| One third of the water | About 67% | ≈ 80 mg/L |
| Half of the water | About 50% | ≈ 60 mg/L |
| Two thirds of the water | About 33% | ≈ 40 mg/L |
Partial drain-and-refill in stages is usually kinder to the pool shell and the water bill than one dramatic emptying — and a full drain is a structural decision, not a chemistry one, especially for in-ground pools. Retest after refilling and rebalance the water, because the fresh water changes alkalinity and hardness too.
What about chemical shortcuts? Cyanuric acid reducer products exist. Results in real pools are inconsistent, and an honest assessment is that dilution remains the dependable fix — it is what we recommend for any pool you are answerable for. Be equally sceptical of anything promising to remove stabiliser through the filter unaided: standard filtration does not touch it.
For an outdoor pool, roughly 30 to 50 mg/L captures most of the sunlight protection on offer. Public-health codes set ceilings, not targets: the CDC's Model Aquatic Health Code caps cyanuric acid at 100 mg/L and requires closure when the ratio of cyanuric acid to free chlorine exceeds 45 to 1. An indoor pool needs no cyanuric acid at all.
Dilution. Drain part of the pool and refill with fresh water — replacing half the water roughly halves the cyanuric acid. Chemical reducer products exist but results are inconsistent, so we do not recommend relying on them for a pool you are responsible for. Filtration and evaporation do not remove cyanuric acid.
No. Chlorine lock is a nickname, not a real lock. High stabiliser slows chlorine down; it does not destroy it. Reduce the cyanuric acid by dilution and the same chlorine residual goes back to working normally.
"Chlorine lock" is the pool-trade name for a pool that will not come right no matter how much chlorine you add. The real mechanism is high cyanuric acid: it shifts so much of the free chlorine into the protected, slow-acting form that the active concentration cannot keep up with the pool's demand. The test kit still shows a healthy free chlorine reading, but algae and bacteria reproduce faster than the chlorine kills them.
Monthly is reasonable for a tablet-fed pool, logged so you see the trend before it becomes a drain-and-refill. Have a high poolside reading confirmed by a laboratory before you pay to move water, and read the result alongside your free chlorine: at 60 mg/L of cyanuric acid, a 1 mg/L chlorine residual is not the protection it would be in unstabilised water.
We will test cyanuric acid alongside the full water balance, and tell you plainly whether dilution is needed and how much.