Pool Stabiliser (CYA) Levels Explained
Stabiliser — also called cyanuric acid, CYA, or conditioner — is one of the most misunderstood pool chemistry parameters. Getting it wrong in either direction causes real problems.
What stabiliser does
Sunlight destroys chlorine quickly. Without stabiliser, an outdoor pool in direct sun can lose most of its free chlorine within 2–4 hours. Stabiliser (cyanuric acid) forms a weak bond with chlorine that shields it from UV degradation, so chlorine lasts much longer.
The tradeoff: that same bond reduces how fast chlorine can kill bacteria and algae. The more stabiliser you have, the more chlorine you need to keep the water safe. At very high stabiliser levels, normal chlorine amounts stop working altogether — even if the reading looks fine.
The right range: 30–50 ppm
The target for outdoor residential pools is 30–50 ppm (parts per million). This gives meaningful protection against UV without significantly weakening chlorine.
Indoor pools do not need any stabiliser — there is no sunlight to protect against.
Why high stabiliser is a serious problem
As stabiliser rises, chlorine needs to rise with it to remain effective. At high levels, keeping chlorine high enough to actually sanitise the water becomes impractical:
Minimum chlorine = 7.5% of stabiliser. Shock target = 40% of stabiliser. Based on TFP SLAM methodology.
Above 80 ppm stabiliser
The water can appear clear while bacteria and algae are not actually being controlled. A pool at stabiliser 100 ppm needs 7.5 ppm chlorine just to function — most domestic test strips do not even measure that high. A partial drain is usually the only practical fix.
How stabiliser builds up
The main cause is trichlor tablets — by far the most popular way to chlorinate a pool. Each tablet is around 57% cyanuric acid by weight. As it dissolves, it adds both chlorine and stabiliser to the water.
A typical 200g trichlor tablet adds roughly 3 ppm stabiliser to a 10,000-gallon pool. Used regularly through a season, stabiliser can climb from zero to 100 ppm or more.
Unlike chlorine, stabiliser does not break down or evaporate. The only way it leaves the pool is through water being removed — backwashing, splashing out, or the fresh water used to top up after evaporation.
How to lower stabiliser
No chemical removes stabiliser reliably.
Products marketed as stabiliser reducers have inconsistent real-world results. The reliable fix is dilution: drain part of the pool and refill with fresh water.
Replacing 25–33% of the water reduces stabiliser by roughly the same proportion. For very high levels you may need a larger drain or multiple partial drains.
Before draining, check your pool type
Vinyl liner pools: do not drain below about 1 foot of water in the shallow end without professional advice — the liner can shrink, wrinkle, or shift when empty.
Fibreglass and concrete pools: draining is generally safer, but avoid emptying completely if groundwater is high — the pool can float upward out of the ground.
A partial drain of 25–33% carries minimal risk for most pool types.
After draining, switch to liquid chlorine or Cal-Hypo granules for top-ups — these do not contain stabiliser. Return to tablets only for routine maintenance once levels are back in the 30–50 ppm range, and test monthly.
Check your stabiliser dose in PoolScan →Warning signs of high stabiliser
- ⚠Chlorine reads normal (1–3 ppm) but the pool looks hazy or algae keeps returning.
- ⚠You need to add large amounts of chlorine just to keep the reading up.
- ⚠Shock treatment has little visible effect even after adding the full dose.
- ⚠Stabiliser test reads above 80 ppm.
PoolScan adjusts chlorine and shock doses for your stabiliser level — and warns you when stabiliser is too high to shock effectively.
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