How to lower stabilizer (CYA) in a pool
Stabilizer (cyanuric acid, CYA) protects chlorine from sunlight, but too much of it makes chlorine sluggish and pushes up the chlorine level you need. CYA doesn't evaporate and breaks down only slowly, so the dependable way to lower it is to replace water. First, make sure the reading is real.
Why high CYA is a problem
The more stabilizer in the water, the more of the chlorine it holds in reserve, and the less is active against algae and germs. A widely used guideline is to keep free chlorine at least 7.5% of CYA:
| CYA | Minimum free chlorine | Shock level (40% of CYA) |
|---|---|---|
| 50 ppm | 3.8 ppm | 20 ppm |
| 100 ppm | 7.5 ppm | 40 ppm |
At 100 ppm, you'd need twice as much chlorine every day to get the same effect, and shocking takes a very large dose. That's why pools with high CYA tend to turn cloudy or green even when the chlorine reading looks "normal". The chlorine and stabilizer guide explains it in more detail.
Check the reading first
Most CYA tests measure how cloudy the sample turns, and they're hard to read precisely at the top of their range. Many top out around 100 ppm, so any higher reading looks the same.
To check a high reading, dilute the sample: mix equal parts pool water and tap water (with no stabilizer in it), test that, and double the result. If both readings agree, you can trust the number. Store tests are another option, but ask which method they use.
How to lower it: replace water
Since fill water normally has no stabilizer, the share of water to replace is simple:
Example: a 15,000-gallon pool reads 100 ppm and the target is 50 ppm. (100 − 50) ÷ 100 = 0.5, so you'd replace half the water, about 7,500 gallons. The stabilizer calculator gives the same answer and the new chlorine minimum: 3.8 ppm instead of 7.5 ppm.
A few practical points:
- Do it in steps if draining half would lower the water too far. Drain, refill, mix for a day, retest, and repeat. The drain and refill calculator plans each step.
- Check your builder's or liner maker's advice before lowering water far. Some pools can be damaged when drained, especially vinyl liners and pools in areas with high groundwater.
- Check local rules about where pool water can be discharged. Many areas ask that chlorine be low before draining to a street or storm drain.
- Rebalance afterwards. Fresh water changes pH, alkalinity, calcium and salt too. Retest everything once it's mixed.
Other methods
- CYA-reducing products. Some products use microbes or enzymes to break down cyanuric acid. Results vary with temperature, chlorine level and the product, so follow the label closely and retest before relying on them.
- Reverse osmosis. In some areas, mobile services filter pool water through reverse osmosis, which removes stabilizer and other dissolved solids without draining. Availability and cost vary.
- Rain and splash-out. Heavy rain that overflows the pool and normal backwashing lower CYA slowly, but rarely enough on their own.
Stop it building up again
The usual cause is chlorinating with trichlor tablets or dichlor all season. Each 1 ppm of chlorine from trichlor adds about 0.6 ppm of CYA, and from dichlor about 0.9 ppm.
Example: a 15,000-gallon pool that uses 3 ppm of chlorine a day would gain about 54 ppm of CYA in 30 days on trichlor alone, according to the chlorine cost calculator. Over a summer, that adds up fast.
- Use liquid chlorine or cal-hypo for daily chlorine once CYA reaches your target, and save tablets for when you're away.
- Shock with liquid chlorine or cal-hypo, not dichlor.
- Test CYA every month or so during the season, so you catch it before it's high.
Frequently asked questions
Can I just add more chlorine instead?
Yes, as long as you keep free chlorine at the higher level CYA demands, every day. It costs more and shocking takes very large doses, which is why most owners lower CYA instead.
Does CYA go away over winter?
Partly, in some pools, but not reliably. Test in spring before adding any.
What level should I aim for?
About 30–50 ppm is common for outdoor pools chlorinated by hand, and often 60–80 ppm for saltwater pools. Check your salt generator's manual.