Guide
How to lower CYA in a pool
Stabilizer (CYA) only comes down when you replace water. The fraction to drain is 1 − target ÷ current; here is the table, when to split it into rounds, and how CYA got high.
Updated · 3 min read
Cyanuric acid (CYA, stabilizer) comes down when you replace water. The fraction to replace is 1 − (target ÷ current): to go from 100 ppm to 70 ppm, replace 30% of the water. Refill, mix, and retest CYA before dosing anything else, because your chlorine target depends on the new number.
When CYA is too high
| Pool | CYA range (ppm) | Aim for | Replace water above |
|---|---|---|---|
| Liquid chlorine or tablets, outdoors | 30 to 60 | 40 | 80 |
| Salt (chlorine generator), outdoors | 60 to 80 | 70 | 90 |
| Indoor | 20 to 30 | 20 | 50 |
| Hot tub (chlorine) | 20 to 40 | 30 | 50 |
The free chlorine a pool needs rises with CYA. Past these levels, holding enough chlorine gets expensive, and clearing algae gets very hard: shock level is about 40% of CYA. If you need to clear a green pool, bring CYA to 70 or lower first.
How much water to replace
| CYA now → target | Replace | In rounds |
|---|---|---|
| 70 → 40 | 43% | one |
| 80 → 40 | 50% | one |
| 90 → 40 | 56% | two rounds of 33% |
| 100 → 40 | 60% | two rounds of 37% |
| 100 → 70 | 30% | one |
| 120 → 70 | 42% | one |
Exchanges over 50% are split into rounds: drain, refill, mix, repeat. Each round replaces the same share of the already-diluted water, so two rounds of 37% leave the same 40% of the old water as one 60% drain would, without lowering the water as far. The table assumes your fill water has no CYA; test it first to be sure.
How to lower CYA, step by step
- Stop the source. If you chlorinate with tablets (trichlor) or dichlor, switch to liquid chlorine first, or the new water climbs straight back up.
- Test CYA and your fill water. The fill water's calcium, pH and alkalinity shape what you'll need to adjust after the refill.
- Work out the fraction. Divide your target by your current CYA and subtract from 1; split anything over 50% into rounds.
- Drain and refill. On a vinyl liner, keep 12 to 18 inches of water in the shallow end; don't drain a fiberglass pool low.
- Mix and retest before dosing anything else. Run the pump, then retest CYA and the rest. Only topping up chlorine should happen before this retest.
When you add chemicals afterwards, add each one separately, with the pump running.
How CYA got high
Tablets and dichlor carry stabilizer with every dose. Each 1 ppm of chlorine from trichlor tablets adds about 0.6 ppm of CYA, and each 1 ppm from dichlor adds about 0.9 ppm. A pool fed only tablets all season gains CYA every week, which is why tablet pools end up here so often. Liquid chlorine and salt cells add none.
Metric notes
The fractions work the same in litres. Enter your readings in the water check and it works out the drain for your pool, in litres or gallons. In Australia CYA is sold as "stabiliser"; lowering it is always water replacement.
Questions people ask
Is there a product that removes CYA?
Replacing water is the dependable way. Whatever you try, judge it by a CYA test afterwards.
Will draining hurt my pool?
Not if you respect the surface: keep 12 to 18 inches of water in the shallow end of a vinyl pool, don't drain a fiberglass pool low, and split large exchanges into rounds.
My salt pool reads CYA 80. Should I drain?
Not yet. A salt pool's range is 60 to 80, and replacing water only makes sense above 90. Keep free chlorine at the salt-pool target for CYA 80.
Does rain lower CYA?
Only when the pool overflows, and then it carries away some of everything, not just CYA. It's the same dilution, just uncontrolled.
Why does my CYA keep climbing without tablets?
Check what you shock with. Dichlor shock adds 0.9 ppm of CYA for every 1 ppm of chlorine, so weekly bags of dichlor raise CYA through a season.