Why Did My Pool Turn Green Overnight?

You go to bed with a sparkling blue pool. You wake up, pull back the blinds, and it looks like a swamp. It happens more often than you'd think, especially in Perth's warm climate. But why does a pool turn green so fast?

The Short Answer: Algae

That green colour is algae — microscopic plant organisms that are constantly present in your pool, waiting for the right conditions to bloom. When those conditions arrive, algae can double its population every few hours. What starts as a slight haze in the evening can be pea soup by morning.

The 5 Most Common Causes

1 Chlorinator Failure

This is the number one cause I see across Perth. Salt chlorinators produce chlorine continuously, so you don't notice when they fail — until your pool turns green. Common failures include worn-out cells (they last 3–7 years), flow sensor errors, or board malfunctions.

Quick check: Look at your chlorinator display. If it's showing "no flow," "check cell," or producing 0% output, that's likely your problem.

2 Heavy Rain

Perth storms can dump huge amounts of rainwater into your pool. This dilutes your chlorine, throws off your pH, and introduces phosphates and organic matter that feed algae. A single heavy storm can drop your chlorine from safe levels to zero.

Prevention: After heavy rain, test your water and shock if needed. Don't assume your chlorinator will catch up — it often can't.

3 Pump or Filter Issues

Your pump circulates water through the filter and chlorinator. If it's not running enough (or at all), chemicals don't distribute properly and your filter can't remove debris. Dead spots in your pool become algae breeding grounds.

Rule of thumb: In Perth summer, your pump should run 8–10 hours daily. Less than that and you're asking for trouble.

4 High Phosphate Levels

Phosphates are algae food. They come from garden runoff, fertiliser, leaves, and even some pool chemicals. If your phosphates are high, algae grows faster than your chlorine can kill it.

The fix: A phosphate remover drops levels quickly, but you also need to address the source — overhanging trees, garden irrigation hitting the pool, etc.

5 Low Stabiliser (CYA)

Stabiliser (cyanuric acid) protects chlorine from UV breakdown. Without enough, Perth's sun destroys your chlorine in hours. By afternoon, you might have zero free chlorine — and algae moves in overnight.

Ideal range: 30–50ppm for salt pools, 60–80ppm for manually chlorinated pools.

What To Do If Your Pool Is Already Green

Once algae has bloomed, normal chlorination won't cut it. You need a proper recovery:

1. Don't swim. Green pool water can harbour harmful bacteria.
2. Check your equipment. There's no point treating the water if your chlorinator is dead or your pump isn't running.
3. Shock treatment. You'll need significantly more chlorine than normal — often 10x your regular dose.
4. Brush and vacuum. Algae clings to surfaces. Brushing it loose helps chemicals work.
5. Clean to waste. Backwash frequently or vacuum dead algae directly to drain — don't recirculate it.
6. Rebalance. Once clear, rebalance your water chemistry to prevent recurrence.

Preventing Green Pools

Prevention is easier than cure. Here's what I tell my clients:

• Test weekly. At minimum, check chlorine and pH. Catch problems early.
• Monitor your chlorinator. Know what normal output looks like so you notice when it drops.
• Run your pump enough. 8–10 hours in summer, 6–8 in winter.
• After rain, test and adjust. Don't assume it'll sort itself out.
• Keep phosphates low. Under 200ppb is ideal.
• Address equipment issues promptly. A weird noise or "check cell" warning shouldn't be ignored.

When To Call a Professional

If your pool has turned green and you're not confident diagnosing the cause, or if you've tried treating it yourself without success, it's time to call someone. I've recovered hundreds of green pools across Perth — I can usually have yours clear in 48–72 hours and, more importantly, identify what caused the problem so it doesn't happen again.

Got questions? I'm always happy to give free advice over the phone. Call 0401 908 929 or send me a message.