How to Fix a Green Pool
A green pool means algae — and algae means chlorine dropped low enough for it to grow. The fix is always the same: brush, correct pH, then hold a high chlorine level until every trace of algae is dead. Most pools clear in 2–5 days.
Why pools turn green
Algae spores enter every pool constantly — from wind, rain, and swimmers. They only bloom when free chlorine can no longer kill them faster than they grow. The usual causes:
- •Chlorine ran out — a missed week of maintenance, a heatwave, or heavy use.
- •High stabiliser (CYA) locking up the chlorine — the strip shows chlorine present, but too little of it is active.
- •Poor circulation — a dead pump, clogged filter, or short run times leave zones where algae grows undisturbed.
Step by step
- 1Net out debris and brush the walls and floor — brushing breaks up the algae layer so chlorine can reach it. Repeat daily while clearing.
- 2Test the water. pH and CYA matter most here — free chlorine is almost certainly near zero. Scan your strip →
- 3Lower pH to around 7.2 before shocking — chlorine kills algae far more effectively at low pH.
- 4Shock with liquid chlorine or cal-hypo to your CYA-based target — roughly 40% of your CYA level, minimum 10 ppm. Apply at dusk with the pump running. Full shock guide →
- 5Run the pump 24 hours a day while clearing. Clean or backwash the filter whenever the pressure gauge rises — it will clog quickly with dead algae.
- 6Retest chlorine daily (morning and evening if you can) and re-dose to hold the shock level. The water goes from green → grey-blue → cloudy blue as the algae dies.
- 7You are done killing algae when chlorine holds overnight — less than 1 ppm lost between dusk and dawn. After that, the cloudiness is dead algae, and the filter removes it over the next few days.
Which chlorine to use
Liquid chlorine (10%)
The best choice for green pools — fast, adds nothing but chlorine, and easy to dose repeatedly over several days.
Granular Cal-Hypo (65%)
More concentrated and easier to store. Pre-dissolve in a bucket of clean water before adding — always add granules to water, never water to granules.
Skip the "green pool" miracle products
Algaecides and clarifiers treat symptoms, not the cause. Most green pools clear with chlorine, brushing, and filtration alone — and some copper-based algaecides can stain surfaces or turn blonde hair green.
How long it takes
The most common mistake is stopping too early — one shock, water looks better, chlorine drops, and the algae comes straight back. Hold the shock level until chlorine survives overnight.
Keeping it from coming back
- •Test at least weekly in season and keep free chlorine in range for your CYA level. See the maintenance schedule →
- •Check your CYA a few times a season — if it has crept above 80 ppm, chlorine loses effectiveness and algae wins even when the strip "looks fine".
- •Run the pump long enough to turn the water over at least once a day — typically 8+ hours in summer.
Safety
- ⚠Do not swim in a green pool — algae itself is mostly harmless, but the bacteria that accompany it are not.
- ⚠Do not swim during shocking. Wait until free chlorine drops back below 5 ppm (for CYA under 50 ppm) and the water is clear.
- ⚠Never mix different chlorine products, and always add chemicals to water — not water to chemicals.
Scan your test strip and PoolScan calculates your exact shock dose from your pool volume and CYA level.
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