Green pool: how to fix it

Illustration: the same pool, green.

Why it turned green

Algae are single-celled organisms that, as HealthyWA puts it, grow quickly in sunny and warm conditions and can turn a pool green within a few hours. They take hold when free chlorine drops too low, which happens with hot weather, heavy use, rain, a pump that has not run long enough, or a chlorinator that cannot keep up.

Step by step

1. Test and fix pH to 7.2 to 7.8, so the chlorine works. pH
2. Shock with chlorine. Raise free chlorine well above the normal level, following the product's shock dose for your litres. Calculator
3. Brush every surface. Algae cling to walls and steps.
4. Run the filter continuously and clean it as it clogs.
5. Vacuum the dead algae. HealthyWA advises brushing, then vacuuming the settled algae from the floor the next day rather than trying to remove it through the filter.
6. Retest chlorine, pH, alkalinity and calcium before anyone swims.

Maple's tip

Ask your water supplier or a pool technician about phosphates if your pool keeps going green. HealthyWA mentions phosphorus in the supply as something a technician can check.

How green is green?

Slightly green or dull, walls slippery: early algae. Usually fixed with a strong dose of chlorine, brushing and a day or two of filtering.

Clearly green, floor still visible: moderate algae. Shock dose, brush twice a day, run the filter continuously, backwash or clean it often, and expect several days.

Dark green or swampy, floor not visible: heavy algae. The same steps with a larger dose and more patience; it can take a week. In extreme cases, draining and cleaning may be quicker, but ask a pool professional first, because emptying some pools can damage them.

Why algae grows

Algae spores blow into every pool constantly. They take hold when chlorine is too low to kill them, often because the pool was left unattended, the weather was hot and sunny, the stabiliser level was low so the sun burnt off chlorine, the pump ran too little, or heavy rain diluted the chemicals. Phosphates and nitrates from leaves, garden runoff and fertilisers feed algae and can make it come back faster.

Shock dosing explained

Shocking means raising chlorine far above the everyday level for a short time to kill algae and organic matter. For a green pool, the target is much higher than normal; follow the product label and your pool shop's advice for your pool's volume and stabiliser level, because higher stabiliser needs a higher shock level to work. Liquid chlorine is popular for shocking because it does not add calcium or stabiliser. Add it in the evening, so the sun does not burn it off, with the pump running. Our pool chemical calculator works out doses from your pool's litres.

Brushing matters

Algae clings to walls, steps and corners in a slimy film that protects it from chlorine. Brushing breaks the film so chlorine can reach it. Brush every surface, at least once or twice a day while treating, paying attention to shady corners, steps and behind ladders.

The filter during treatment

Dead algae turns the water cloudy grey, then the filter catches it. The filter will clog fast: check the pressure and backwash or clean the cartridge often, perhaps daily. Run the pump around the clock until the water clears. A cartridge may need a chemical soak afterwards to remove algae residue.

Black and mustard algae

Most green pools have common green algae. Black algae forms dark spots that root into the pool surface and need hard brushing with a stainless steel brush (on concrete and pebble only), spot treatment and higher chlorine. Mustard algae is yellowish and clings in shady spots; it often returns from contaminated toys, swimwear and equipment, which should be cleaned too. Ask a pool shop for help with either.

After the pool clears

Once the water is clear and chlorine has fallen back into the normal range, test and rebalance pH, alkalinity and stabiliser. Clean the filter thoroughly. Check that chlorine holds overnight, which shows algae and organic matter have been dealt with. Then keep up regular testing so it does not happen again.

Preventing a green pool

• Keep free chlorine in range every day, especially in hot weather.
• Keep stabiliser in range on outdoor pools.
• Run the pump long enough and keep the filter clean.
• Brush weekly and remove leaves promptly.
• Test after storms and heavy use, and before going away.
• Arrange for someone to check the pool, or set up a chlorinator properly, if you go on holiday in summer.

A day-by-day plan for a green pool

Day 1, evening. Skim out leaves and debris, empty the skimmer and pump baskets, and backwash or clean the filter. Test pH and bring it into range, because chlorine works poorly at high pH. Add a shock dose of chlorine for your pool's volume around the deep end with the pump running. Brush every wall, step and corner. Leave the pump running all night.

Day 2. The water should turn from green to a cloudy blue-grey as algae dies. Brush again, check the filter pressure and backwash or clean it, and test chlorine. If chlorine has dropped to near zero overnight, there is still a lot of algae: add another shock dose in the evening.

Day 3 and after. Keep brushing and filtering. Clean the filter as often as the pressure needs. Vacuum dead algae from the floor, to waste if your filter allows, so it does not clog the filter. Once chlorine holds overnight and the water is clear, stop shocking and let chlorine fall back to the normal range.

Final step. Rebalance pH, alkalinity and stabiliser, give the filter a thorough clean or chemical soak, and resume normal testing.

Algaecide: when it helps

Algaecide can help prevent algae returning and can support treatment of stubborn types, but it is not a substitute for chlorine. Some algaecides foam or stain if overdosed, and some contain copper, which can stain pool surfaces and hair. Use it as directed, after the pool is clear and balanced, rather than as the main treatment for a green pool.

Phosphates

Phosphates come from leaves, garden fertilisers, soil, some tap water and even some pool chemicals. They are food for algae. They do not cause algae by themselves when chlorine is in range, but high phosphates can make algae return quickly when chlorine dips. A pool shop can test phosphates, and a phosphate remover can lower them. Reducing runoff into the pool and removing leaves promptly helps keep them down.

Is a green pool dangerous?

A green pool is unsafe to swim in, because low chlorine lets germs grow and swimmers cannot be seen in murky water. It can also become a mosquito breeding site in summer. Keep the pool fenced and children away until it is clear and sanitised.

How long it takes to clear a green pool

A lightly green pool can be clear in a day or two. A moderately green pool usually takes three to five days of shocking, brushing and filtering. A swampy pool can take a week or more. The filter's size and the effort put into brushing and cleaning it matter more than the amount of chlorine poured in.

Green pool after rain

A storm can tip a borderline pool into green within days: rain dilutes chlorine, lowers stabiliser and washes in soil, leaves and phosphates. After heavy rain, empty the baskets, test chlorine and pH, top up chlorine, run the pump longer and brush. Catching it early saves a full rescue.

Our short answer

Balance pH, shock with chlorine at dusk, brush twice a day, run the pump continuously, clean the filter often, and keep going until chlorine holds overnight and the water is clear. Then rebalance and keep testing.

Green pool versus green water from metals

If the water turns green or blue-green right after adding chlorine, especially when filled from a bore or tank, the cause may be copper or iron rather than algae. Algae feels slimy on walls and is cloudy; metal discolouration is clear and does not feel slimy. Adding more chlorine to metal-tinted water can make it worse. Ask a pool shop to test for metals, and treat with a metal remover or sequestrant if they are present.

Keeping a pool from going green on holiday

Before going away in summer, clean the filter, balance pH, raise chlorine to the top of the range, check the chlorinator output and pump timer, empty baskets and cover the pool if you have a cover. Ask a neighbour or a pool service to check it once or twice, especially after storms. A pool that is balanced and filtering when you leave usually stays clear for a week or two.

Costs of fixing a green pool

The main costs are chlorine, often several large doses, a pH correction, extra electricity for running the pump continuously, filter cleaning products, and possibly a phosphate remover or a filter cartridge replacement afterwards. A pool service rescue costs more but saves time. Preventing it with regular testing is far cheaper than either.

Common questions