Short answer: if your free chlorine tests in range but the water is dull, cloudy or algae keeps returning, test cyanuric acid (CYA). CYA shields chlorine from UV, but the same binding that protects it also slows it down. The industry consensus target for an outdoor chlorine pool is 30 to 50 ppm, and the working rule is that free chlorine should be kept at roughly 7.5% of your CYA level. At 40 ppm CYA that is about 3 ppm free chlorine. At 100 ppm CYA, the same 3 ppm is no longer enough — and no amount of shocking fixes the ratio, because only dilution lowers CYA.
The ratio table
| CYA (ppm) | Minimum free chlorine at 7.5% | Comment |
|---|---|---|
| 0 | — | Unstabilised: chlorine burns off fast in summer sun |
| 20 | about 1.5 ppm | Low but workable; some UV loss |
| 30 | about 2.3 ppm | Bottom of the recommended band |
| 40 | about 3 ppm | A common target for an outdoor pool |
| 50 | about 3.8 ppm | Top of the recommended band |
| 70 | about 5.3 ppm | Drifting high; sanitiser demand climbing |
| 80 | about 6 ppm | Most people are not holding this — and that is when algae appears |
| 100 | about 7.5 ppm | Impractical to maintain; plan a partial drain |
Read that column carefully. The chlorine number that was correct at 40 ppm CYA is half what you need at 80 ppm. Nothing about the pool changed except a slow, invisible accumulation.
Where the CYA came from
Almost nobody adds cyanuric acid deliberately. It arrives with stabilised chlorine:
- Trichlor tablets and dichlor granules both contain stabiliser. Every tablet you float adds CYA that never leaves.
- Stabiliser or conditioner products are pure CYA, sometimes added on autopilot at opening.
- CYA does not evaporate, does not break down meaningfully over a season, and is not consumed by chlorine demand. It only leaves in water you take out — via a partial drain, backwash, or splash-out replaced with fresh fill.
That is why pools drift upward year after year: tablets every week, top-ups instead of dilution, and no one testing CYA because the free chlorine reading looked fine.
Fixing a high CYA reading
- Test CYA properly. The turbidity test is read at eye level in sunlight; rushed readings run low. Confirm with a second test or a shop test before draining anything.
- Work out the dilution. CYA falls in proportion to the water replaced — replacing a quarter of the volume with unstabilised fill cuts CYA by roughly a quarter. Standard guidance for pools above the recommended band is to drain and refill 25 to 50 percent, repeating if needed.
- Check local rules and your pool type before draining. Vinyl liners can float and fibreglass or high-water-table sites can be damaged by a deep drain — drain in stages.
- Change the chlorine source. If CYA keeps climbing, switch from trichlor tablets to liquid chlorine or cal-hypo, which add no stabiliser.
- Re-balance after refill. Fresh fill water changes pH, alkalinity and calcium hardness as well.
If CYA is fine, it is the filter
Cloudy water with correct, in-ratio chemistry is a mechanical problem. Work down this list:
| Check | What you are looking for | Fix |
|---|---|---|
| Filter pressure vs clean baseline | 8–10 psi above clean start pressure | Clean the cartridge or backwash |
| Cartridge condition | Flattened pleats, oily grey film, torn ends | Deep-soak, or replace — cartridges are consumables |
| Run time | Whole volume should turn over daily | Increase pump hours before adding any chemical |
| Sand or DE media | Channelling, old sand, missing DE charge | Recharge DE after every backwash; replace sand periodically |
| Pump basket and skimmer | Restricted flow, air in the lid | Clear baskets; check for a suction air leak |
| Return jets | Dead spots with no circulation | Aim jets to rotate the whole pool, angled slightly down |
| Calcium hardness and pH | Scale-forming water clouds up | Re-balance before chasing it with clarifier |
Clarifiers and flocculants are the last step, not the first. A clarifier clumps fine particles so the filter can catch them — which does nothing if the filter is the problem.
Quick answers
Can I lower CYA with a chemical? Products exist, but partial drain and refill is the standard, reliable method. Dilution is the only mechanism you can calculate in advance.
Does CYA matter in an indoor pool? Much less — without UV there is little to protect chlorine from, so stabiliser mostly just slows the chlorine down.
Do saltwater pools need CYA? Yes. A salt chlorine generator still makes chlorine that sunlight destroys, so the same stabiliser logic applies.
Why does algae keep coming back after shocking? Usually because the free chlorine is falling back below the CYA-adjusted minimum within a day. Fix the ratio, not the symptom.
How often should I test CYA? Monthly through the season is plenty, plus once at opening and once after any large refill.
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