Illustration.
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.
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.