Cyanuric acid: the stabilizer that protects chlorine from the sun
Stabilizer (cyanuric acid) stops the sun from breaking down chlorine within a few hours. A little helps a lot, usually 30–50 mg/l. Too much holds chlorine back, and it only goes away with a water change.
What stabilizer does
The sun's ultraviolet radiation breaks down free chlorine. In an outdoor pool without stabilizer, a summer midday can wipe out most of the chlorine within a few hours, even if nobody swims. Cyanuric acid binds loosely to chlorine, protects it from the sun and lets it be released gradually as it is needed.
The trade-off is that chlorine “held” by stabilizer works more slowly. The higher the stabilizer, the more free chlorine you need for the same result. An indoor or permanently covered pool usually does not need any at all.
How much you need
| Cyanuric acid | What it means |
|---|---|
| 0–20 mg/l | Chlorine is lost quickly in the sun, high consumption |
| 30–50 mg/l | The usual target for an outdoor pool |
| 50–80 mg/l | It works, but keep free chlorine towards the upper limit |
| Above about 80–100 mg/l | Chlorine is held back; algae appear while the chlorine test reads “correct” |
With salt chlorination, the desired range is given by the unit's manufacturer.
How it builds up
Stabilizer does not evaporate, is not used up and is not caught by the filter: whatever goes in, stays. It gets in either deliberately, as granules or liquid at the start of the season, or without you noticing, through stabilized pool chlorine. Trichlor tablets and dichlor granules leave cyanuric acid behind with every dose, so a pool chlorinated only with tablets starts the season at the right level and often ends it well above.
Granular stabilizer dissolves slowly and is acidic. Follow the instructions on the packaging for dissolving it, and never let it sit in a pile on the liner.
When there is too much stabilizer
The signs: chlorine tests within range, but the water turns dull, algae keep coming back and shock chlorination does not give the expected result. Since no chemical removes it reliably, the answer is dilution:
- Measure the stabilizer with a turbidity drop test kit or a photometer; strips are inaccurate for this test (see how to test the water).
- Work out how much water needs changing: if you replace a quarter of the water, the stabilizer drops by about a quarter.
- Make the change in stages, lowering the level a little each time and refilling. A liner pool is never emptied completely; the details are in the article why we don't empty the pool.
- For the rest of the season, chlorinate with unstabilized chlorine, e.g. liquid, so the level does not climb again.
Filter backwashes and rain that makes the pool overflow help a little, because water is lost, but they are rarely enough on their own.
A simple plan for the season
- At opening: test the stabilizer and, if it is low, bring it to about 30–40 mg/l.
- During the season: test it about once a month, not every week, because it changes slowly.
- When it approaches 50–60 mg/l: switch to liquid chlorine or another form without stabilizer, before it reaches a level that needs dilution.
Questions
Do I need to add stabilizer every year?
Only if the test when you open the pool shows a low level. After a rainy winter it may have dropped, whereas if you chlorinate with trichlor tablets you usually do not need to add any more.
Is there a product that removes stabilizer?
There are products on the market that promise to reduce it, but their effect depends heavily on conditions and is not guaranteed. A partial water change remains the most reliable method.
See also
Pool chlorine
Pool chlorine as tablets, granules or liquid: which form suits you, what free and combined chlorine are, and…
Shock chlorination
Pool shock chlorination: when you need it (storms, many swimmers, cloudy water), how to do it without harming…
Why we don't drain
Why a liner pool is never drained: what happens to the membrane without water, how groundwater affects it and…