A green pool can happen faster than most people expect. One warm weekend, a bit of rain, and chlorine that has dropped lower than it should be is often all it takes. But don't worry, it is fixable and once you know what caused it, it is easy enough to stop it happening again.
Why Your Pool Turns Green
Green water is almost always caused by algae. Algae bacteria are naturally present in the environment and find their way into your pool through rain, wind and debris.
Under normal conditions your chlorine keeps them under control. When something in that system slips, algae can take hold quickly, sometimes turning your pool green within 24 to 48 hours.
The most common reasons it happens:
| Cause | What Goes Wrong |
|---|---|
| Chlorine levels dropping too low | Without enough sanitiser, algae establish and spread fast |
| pH drifting out of range | Chlorine becomes less effective even when levels look normal |
| Filter not keeping up | Debris and contaminants build up and feed algae growth |
| Not enough pump run time | Stagnant water in low-flow areas gives algae a place to settle |
| Heavy rain or debris | Dilutes your chlorine and introduces nutrients into the water |
| Irregular brushing | Algae attach to surfaces and spread if not physically disturbed |
How to Fix a Green Pool
Step 1: Test Your Water Before You Do Anything
Before adding chemicals, you need to know where your water is sitting. Shocking a pool with the wrong pH wastes product and slows the whole process down.
For most pools, aim for these ranges before you start treatment:
pH between 7.2 to 7.6
Total alkalinity between 80 and 120 ppm
Adjust these first if they are significantly off, then move to shocking.
Step 2: Shock the Pool
Shocking raises your chlorine levels quickly to kill off algae. For a green pool you will need a higher dose than a regular maintenance shock.
Check your product label for the correct dosage based on your pool volume and the severity of the algae, as this varies between products.
Add the shock in the evening with your pump running so UV does not break it down before it can do its job. Keep the pump running continuously until the water clears.
Step 3: Brush the Entire Pool
Do not skip brushing. Algae clings to walls, the floor, steps and corners, and shocking works much better when you dislodge them into the water first.
Brush thoroughly before and after adding shock, paying extra attention to shaded areas and anywhere water flow is limited.
Step 4: Keep Your Filter Clean Throughout
This is where most people underdo it. Your filter is pulling dead algae out of the water as the shock works through the pool. If it gets blocked during treatment, the whole process slows down.
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Sand filters: Backwash when pressure rises above your normal operating range, and again after the initial treatment.
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Cartridge filters: Rinse thoroughly and repeat during treatment if needed.
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DE filters: Backwash and recharge with fresh DE powder after the treatment cycle.
Keep checking and cleaning your filter until the water is fully clear.
Astral Pool XC 75 Cartridge Filter
Step 5: Vacuum the Algae Out
Once the algae has settled on the floor, vacuum it out.
If your filter system has a "waste" setting on the multiport valve, use that so the debris goes straight out of the system rather than back through your filter.
No waste setting? Vacuum as normal and clean your filter straight afterwards.
Step 6: Test Your Water Again and Fix What Is Off
Once the water has cleared, grab your test kit and check your water again.
Treating a green pool puts your water through a lot, and your pH, alkalinity and chlorine levels will often shift during the process. Do not assume the water is ready just because it looks clear.
Why Your Pool Keeps Turning Green
If this is happening more than once a season, one of these is usually the reason:
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Not enough pump run time: If your water is not turning over fully each day, low-flow areas become a breeding ground for algae. Check your daily run time against your pool volume.
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Filter maintenance has slipped: A filter losing efficiency over time is one of the most common causes of recurring algae.
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Chlorine dropping faster than expected: An uncovered pool loses chlorine quickly in hot weather. A pool cover helps maintain levels between treatments. Check out our guide on regular pool cleaning and maintenance.
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The salt cell needs attention: If you run a salt chlorinator and the pool keeps going green, check the output reading on your controller and inspect the cell for scale buildup.
Astral Pool VX Salt Chlorinators
Frequently Asked Questions
How long does it take to fix a green pool?
For a mildly green pool with the right treatment approach, most people see clear water within two to four days. A heavily green pool can take longer depending on how severe the algae bloom is and how well your filter keeps up during treatment. Keeping your filter clean throughout makes a significant difference to how quickly the water clears.
Can I swim in a green pool?
No. Green water means algae is active and your chlorine levels are likely too low to sanitise the water properly. Wait until the water is fully clear, your chlorine is back in the normal range, and your pH is balanced before getting back in.
How do I stop my pool turning green after rain?
Rain dilutes your chlorine and can shift your pH, which is why pools often turn green after a storm. Test and rebalance your water after any significant rainfall, give the pool a maintenance shock, and check your filter is running properly. A pool cover helps keep rain and debris out between uses.