If your routine looks like “add chlorine today → test tomorrow → back to zero,” the problem is often not the product. In many outdoor pools, the underlying issue is low CYA (stabiliser), meaning sunlight can destroy free chlorine faster than your schedule replaces it. The practical fix: raise stabiliser in stages, then keep a realistic FC target for the new CYA.
- FC is hard to hold: you dose often but readings keep dropping.
- Big daytime loss: morning FC looks OK, late-day FC is much lower.
- Repeated “top-ups” don’t settle it: you get short relief, then the same crash.
- Algae pressure rises: dullness/haze after a hot weekend, then green patches.
- Salt pool can’t keep up: longer runtime, higher % — still a downward trend.
Free chlorine (FC) is your pool’s active sanitizer. Outdoors, sunlight breaks down FC. CYA (cyanuric acid) acts like a buffer: it “holds” part of chlorine in reserve and slows UV destruction. That doesn’t make chlorine weaker by default — it makes the daily level more stable.
In practice, warm water increases biological activity (algae pressure), while UV increases chlorine loss — a double hit if stabiliser is close to zero. That’s why owners often say: “I’m adding chlorine every day and it still reads zero.”
If your pool is outdoors and gets meaningful sun, you typically want some CYA so chlorine survives daytime UV and doesn’t crash daily.
Deeper explainer: CYA & sun protection.
A workable CYA target depends on two variables: (1) how much direct sun hits the water, and (2) how chlorine is generated (liquid dosing vs salt chlorinator). That’s why “one universal number” fails.
| Pool setup | Suggested CYA strategy | Notes |
|---|
If you’re tuning a salt setup: Chlorination & salt systems.
Stabiliser is easy to add but slower to show up on tests. The three common mistakes are: (1) adding too fast, (2) retesting too soon and adding again, and (3) raising CYA but leaving FC targets unchanged.
10 ppm in 10,000 L ≈ 100 g.
If your product is a blend, follow label rates and adjust calculations accordingly.
| Pool volume | +10 ppm | +20 ppm | Notes |
|---|
- Measure the dose, put stabiliser into a clean sock/stocking, tie it off.
- Hang it in front of a return jet (or place in the skimmer basket if your system supports it).
- Keep circulation running; squeeze the sock occasionally to speed dissolving.
- Avoid backwashing / major filter cleaning immediately after dosing (don’t lose product).
Raising CYA but leaving FC too low. Once CYA increases, the pool typically needs a higher FC target to remain equally stable. The goal is steady protection, not just “any number on the strip.”
CYA doesn’t always show on a test immediately. Testing too soon is the main reason people add twice and overshoot. Use a practical schedule and observe the FC trend over several days.
- 48–72 hours: an early check if most product has dissolved and circulation has been running.
- ~5–7 days: often the most reliable confirmation point for the new CYA level.
- If the sock is still dissolving: wait until it’s fully gone before judging the number.
Usually yes. A cover reduces UV exposure and can slow chlorine loss. But it doesn’t make sanitizer optional. If the cover is off for part of the day, you still want enough CYA to keep FC stable during sun exposure.
Often, yes. Salt chlorinators can benefit from higher CYA because it reduces UV burn-off and helps maintain a steadier FC baseline. The important part is matching your daily FC target to the chosen CYA and confirming the system can keep up.
You can — but track CYA periodically. Trichlor/dichlor products add stabiliser over time, and CYA can climb quietly if tablets become the default.