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What it does
Ultraviolet light in sunlight breaks chlorine down quickly. Stabiliser (cyanuric acid, also called isocyanuric acid) shields chlorine from UV, so an outdoor pool needs far less chlorine. HealthyWA recommends it in outdoor pools that use chlorine, at 30 to 50 mg/L, and not in spas or indoor pools.
How much to add
1 mg/L is 1 gram per 1,000 litres. Raising a 50,000 L pool from 20 to 40 mg/L takes 20 × 50 = about 1,000 g of stabiliser (at 100%). Calculate yours. Dissolve it as the label directs (often in the skimmer or a sock in the skimmer, with the pump running) and wait several days before retesting, as it dissolves slowly.
Why it builds up
Stabilised chlorine products (dichlor powder and trichlor tablets) add stabiliser every time you use them. HealthyWA warns it can build up to the point that water has to be drained or backwashed to bring it down.
Bringing a high level down
Dilution with fresh water reduces the concentration: replacing a quarter of the water lowers it by about a quarter when the replacement water contains no stabiliser. Ask a pool professional before draining. Groundwater pressure, the shell and lining, discharge rules and water restrictions can make it unsuitable; follow the pool manufacturer and local water authority requirements.
With stabiliser in the water, keep free chlorine higher: HealthyWA's minimum for a stabilised outdoor pool is 2 mg/L, or 3 above 26°C.
Indoor pools and spas
Indoor pools do not get direct sunlight, so stabiliser is not needed and can make chlorine slower to act. Most guidance says not to use it indoors, or in spas, unless your pool professional advises otherwise.
How to add stabiliser
Stabiliser dissolves slowly. Many labels suggest dissolving it in a bucket of warm water, or placing it in a sock or the skimmer basket with the pump running, so it does not sit on the pool floor and mark the surface. Do not backwash for a few days afterwards, so it is not washed out before it dissolves. Retest after about a week before adding more.
Testing stabiliser
Most home test kits use a cloudiness test for cyanuric acid, read against a scale. It is less precise than chlorine and pH tests, so test in good light and repeat if unsure, or ask a pool shop to test a sample.
Stabiliser and chlorine together
Stabiliser and chlorine work as a pair. As stabiliser rises, chlorine needs to be kept higher to work just as well, because stabiliser binds part of the chlorine while shielding it from sunlight. That is why guidance gives a higher minimum free chlorine for stabilised outdoor pools, and why very high stabiliser makes a pool hard to keep clear. Keep stabiliser in the recommended range and chlorine at the matching level, and test both regularly through summer.
Our short answer
For an outdoor pool, keep stabiliser in the recommended range, add it slowly, retest after a week, and never use it indoors. If it climbs too high, dilute with fresh water after talking to a pool professional.
How fast stabiliser is lost
Stabiliser does not evaporate or burn off; it falls only through dilution from backwashing, splash-out and rain, so levels change slowly.
Stabiliser and salt pools
Salt pools outdoors also need stabiliser, because the chlorine the cell makes is broken down by sunlight just like added chlorine.
Too much stabiliser: signs
If the pool keeps going cloudy or green despite good chlorine readings, high stabiliser may be the cause; test it.
Testing after adding
Retest stabiliser about a week after adding it, once it has fully dissolved and mixed.
Common questions
What should stabiliser be in a pool?
30 to 50 mg/L in an outdoor pool, according to HealthyWA.
Do salt pools need stabiliser?
Outdoor salt pools benefit from it too, because the chlorine the cell makes is broken down by sunlight like any other.
How do you lower stabiliser in a pool?
Fresh-water dilution lowers the concentration, but ask a pool professional to check structural, groundwater and discharge conditions before draining.