Have you ever noticed your pool chlorine disappearing fast, or the water turning cloudy even though the readings look fine?

Often the culprit isn't the chlorine itself but a key part of water chemistry: stabiliser (cyanuric acid). Understanding and controlling it is the key to using fewer chemicals and keeping the water right all season.

What is stabiliser and why do you need it?

Free chlorine is very sensitive to ultraviolet light. Without protection, the sun can destroy up to 90% of residual chlorine in about two hours.

Stabiliser acts as a shield: it binds to the chlorine and stops the sun from breaking it down (photolysis). Chlorine stays active for longer, so disinfection is more efficient and cheaper.

Key fact: stabiliser builds up. It doesn't evaporate with the water: it stays in the pool until the water is replaced.

Working ranges: getting the balance right

To keep the water safe without wasting product, stabiliser should stay within the recommended range (measured in ppm, parts per million).

🟢 Optimal (20–50 ppm). Maximum efficiency. Chlorine is protected from the sun but still available to fight bacteria and algae. The water stays clear with less product.

🔴 Critical / over-stabilised (above 70 ppm). This is "chlorine lock": too much cyanuric acid ties up the chlorine so it can't disinfect. Result: cloudy water, algae and strong smells (chloramines), even when the test shows plenty of chlorine.

🟠 Too low (below 20 ppm). Chlorine is unprotected, breaks down fast and has to be topped up constantly.

How to fix it

Case A: level too low (add stabiliser)

Common after filling the pool or after heavy rain.

Use granular stabiliser (cyanuric acid).
Pre-dissolve it in a bucket of warm water so it mixes in properly and doesn't stain the lining.
Pour it into the skimmer with the filtration running.
Keep the water circulating for at least 24 hours.

Case B: level too high (reduce it)

Typical at the end of the season after regular use of trichlor (multi-function tablets).

Technical note: there is no effective chemical to remove excess stabiliser. The only fix is a partial water change:

Drain roughly a third of the pool volume.
Refill with fresh mains water (which contains no stabiliser).
Test again after circulating.

The link with pH, and a summary

Stabiliser only works well if pH stays between 7.2 and 7.6. The right stabiliser level also helps keep pH steady (buffering effect).

Test: check cyanuric acid monthly (weekly in high season).
Adjust: keeping it between 20 and 50 ppm is the priority.
Prevent: if you use stabilised chlorine tablets, plan partial water changes so it doesn't build up.

Pool stabiliser: a practical guide to testing and control