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Pool stabiliser: what cyanuric acid is for

Stabiliser is chlorine’s bodyguard against the sun. Without it, chlorine evaporates in a few hours; in excess, it smothers it. It is all a matter of the right dose.

In short

Stabiliser, or cyanuric acid, protects chlorine from the UV rays that destroy it very fast in full sun. At the right dose (30 to 50 mg/L), it clearly extends chlorine’s action. But above 70 mg/L, it "locks" the chlorine, which becomes ineffective even when well dosed. Since it builds up with stabilised chlorine, it needs watching.

Its role: protecting chlorine

In full sun, unprotected chlorine loses much of its effectiveness within hours. Stabiliser acts as a screen: it slows this destruction and lets chlorine last the day. It is especially useful for very sunny outdoor pools.

The trap of excess

Stabiliser is barely consumed and builds up with every stabilised-chlorine tablet. Above 70 mg/L, it holds the chlorine back too much and it stops working: the water can turn green despite a correct dose. The only way to bring it down is to renew part of the water — hence the value of alternating with unstabilised chlorine.

What to do, step by step

  1. Measure your stabiliser

    Aim for 30 to 50 mg/L. Regular testing prevents silent build-up.

  2. If it is too low

    Add stabiliser up to target, especially for a very sunny pool.

  3. If it is too high

    Renew part of the water and switch temporarily to an unstabilised chlorine.

  4. Watch it over time

    With stabilised tablets, recheck during the season: the level creeps up slowly but surely.

Frequently asked questions

What is the right stabiliser dose?

Between 30 and 50 mg/L. Below 20, chlorine is destroyed too fast by the sun; above 70, it becomes ineffective.

Why does my stabiliser rise on its own?

Because stabilised chlorine (most tablets) adds some with each dose, and it barely degrades. So it accumulates over the season.

Stabiliser and chlorine — what is the difference?

Chlorine disinfects; stabiliser does not disinfect but protects chlorine from the sun. They are two complementary roles.