What Shocking Actually Does
Weekly chlorine keeps algae and bacteria in check. Shocking raises free chlorine high enough (typically 10 ppm or more) to oxidise combined chlorine — the smelly, irritating stuff people think is "too much chlorine". It's usually the opposite: you can smell it because there isn't enough free chlorine left.
The three common types in Australia:
- Calcium hypochlorite (cal hypo) — granular, fast, cheap. Adds calcium. Don't use it in vinyl liners without dissolving it fully, and go easy if your hardness is already high.
- Liquid chlorine (sodium hypochlorite) — easy to pour, no calcium, shorter shelf life. My default for most concrete pools.
- Non-chlorine shock (potassium monopersulphate) — oxidises without raising chlorine. Useful for spas and when you want to swim sooner. It does not kill algae on its own.
When to Shock
- The water looks dull or has a slight haze
- You can smell chlorine, or eyes are stinging
- After a heatwave, a party, or heavy rain
- After a green-pool treatment, as a follow-up
- As a weekly summer habit if the pool is used a lot
You don't need to shock every week in winter if the water is clear, chlorine is holding, and nobody is swimming.
How Much to Add
For a typical 50,000 litre pool, as a starting point:
| Situation | Granular shock | Liquid chlorine |
|---|---|---|
| Maintenance / after a party | 500 g – 1 kg | 5 – 10 L |
| Cloudy or smelly | 1 – 2 kg | 10 – 15 L |
| Green pool (see green-pool guide) | 2 – 5 kg | 15 – 30 L |
Always read the bag — strengths vary. When in doubt, add half, wait overnight, test, then add more.
How to Do It
- Test and fix pH first. Shock into water above pH 7.8 and most of it is wasted. Target 7.2–7.4.
- Wait until dusk. Sunlight burns chlorine off in hours. Night-time shock works overnight.
- Dissolve granular shock in a bucket of pool water — never dump dry granules in one spot, especially on a liner or dark pebble.
- Pour around the deep end, away from the skimmer. Keep the pump running.
- Brush the walls and floor so the chlorine reaches every surface.
- Run the filter overnight. Clean or backwash in the morning if pressure has jumped.
Safety: Gloves, eye protection, old clothes. Never mix shock with acid or algaecide in a bucket. Don't swim until free chlorine is back under 5 ppm (ideally 1–3 ppm).
After You Shock
Next morning, test free chlorine, pH and alkalinity. Expect pH to have moved. Let chlorine fall naturally with the pump running — don't try to "neutralise" it unless you urgently need the pool that day. Then get back on your normal weekly dose.
Common Mistakes
- Shocking at midday — UV destroys it before it can work.
- Dumping granules in one pile — bleaches vinyl and stains pebblecrete.
- Ignoring pH — the most expensive way to do nothing.
- Swimming the same evening — wait for the test, not the clock.
- Using non-chlorine shock on a green pool — it won't kill the algae. You still need chlorine.
