Why does my chlorine drop to zero so fast?

Chlorine demand: algae, organics, or heavy bather load consume it faster than you add it. Sun also burns it off quickly when stabiliser (CYA) is low. A dose that disappears within hours means there's a demand to shock out.

How do I fix chlorine that won't hold?

Balance pH to 7.2–7.6, then shock to 10–15 ppm and hold it with the filter running until chlorine stops crashing. Clearing the demand is the fix; small top-ups alone won't keep up with it.

Could low stabiliser be the cause?

Partly. Cyanuric acid (CYA) shields chlorine from sunlight; with little or none, an outdoor pool can lose most of its free chlorine in a sunny afternoon. If chlorine only vanishes during the day, low CYA is a likely factor alongside any demand.

Low chlorine

Why your chlorine won't hold

You dose to the right level, test a few hours later, and it's back to zero. That disappearing act has a name — chlorine demand— and once you clear it, chlorine starts sticking again.

What's eating it

Free chlorine gets consumed by whatever it's sanitising. When there's a lot to fight — an early algae bloom, organic load from heavy use, sunscreen and sweat, or debris after a storm — it burns through a normal dose in hours. Strong sun also strips it fast if your stabiliser (CYA) is low. A reading that vanishes quickly is the signal there's a demand to clear, not just a top-up to add.

How to fix it

1. Balance pH (7.2–7.6) so the chlorine you add actually works.
2. Shock past the demand.Raise free chlorine to shock level (10–15 ppm) and hold it with the filter running — that's what burns through the backlog a normal dose can't.
3. Confirm it holds. Once chlorine stops crashing and settles, the demand is cleared. Then keep it at 1–3 ppm.

Work out your exact dose

Numbers in hand? This free calculator turns them into the exact amount for your pool's size:

Pool Shock CalculatorHow much shock to add to reach shock level and clear the water.

Frequently asked

Why does my chlorine drop to zero so fast?: Chlorine demand: algae, organics, or heavy bather load consume it faster than you add it. Sun also burns it off quickly when stabiliser (CYA) is low. A dose that disappears within hours means there's a demand to shock out.
How do I fix chlorine that won't hold?: Balance pH to 7.2–7.6, then shock to 10–15 ppm and hold it with the filter running until chlorine stops crashing. Clearing the demand is the fix; small top-ups alone won't keep up with it.
Could low stabiliser be the cause?: Partly. Cyanuric acid (CYA) shields chlorine from sunlight; with little or none, an outdoor pool can lose most of its free chlorine in a sunny afternoon. If chlorine only vanishes during the day, low CYA is a likely factor alongside any demand.

More pool help

Common pool problems — Green, cloudy, low chlorine, drifting pH — what's wrong and how to fix it.
Why your pool turned green — Green water is algae from low chlorine — the step-by-step fix and how long it takes.
How much chlorine to add — Hit a safe 1–3 ppm — the rule of thumb, worked examples, and why it won't hold.
How to lower your pool's pH — Pool pH too high? Why it matters and how to bring it back to 7.2–7.6.
How to raise your pool's pH — Pool pH too low? Acidic water stings and corrodes — how to bring it back up.
Cloudy pool water — Clear but not green? The three usual causes and how to get back to clear.
How much shock to add — Reach shock level (10–15 ppm) to clear the water — what it means and how much.

Stop guessing, start swimming

Algae Later reads your pH and chlorine, tells you exactly what to add, and nudges you when it's time to test again.

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