How to treat green water in your pool? A step-by-step guide

In summer, it's not uncommon for pool water to turn green. There are several reasons for this. It can be the result of particular climatic conditions (heatwave, rain/storms) that alter the balance of the water. Similarly, poor maintenance or filtration can lead to the development of green algae. In this article, you'll find practical advice on how to restore clear water with the right treatments and maintenance tips.

Green pool water with or without algae

Why does your pool water turn green?

Climatic factors that affect water

Hot weather / heatwave

During a heatwave, the water in your swimming pool overheats without cooling down during the night, thanks to the sun's UV rays. As soon as the water temperature rises above 28°C, bacteria multiply. The water then turns green. Pool walls become slippery. Without rapid action, green algae will appear. React as soon as the water becomes cloudy!

Rain and storm

Rainwater is loaded with fine particles such as pollen, dust and sand. What's more, it has a pH between 5 and 6, whereas your pool water is balanced between 7 and 7.4.

Technical issues

Poor filtration

Faulty filtration due to pump problems or clogged filters is responsible for 80% of green water. Secondly, too short a filtration time also alters the balance of the water, which will turn green before algae develop. That's why it's important to know how much time you need to activate filtration !

Too little maintenance

Feuilles, sable, pollen…l’eau de votre bassin se charge de saletés, surtout si elle est en extérieur ! Il faut donc contrôler régulièrement l’équilibre de l’eau, son pouvoir désinfectant mais aussi la nettoyer très régulièrement.

Human factor

The more bathers there are in a pool, the more the water deteriorates. Sunscreen, sweating...the water is rapidly polluted by the presence of bathers, who bring in organic matter. A quick rinse before diving in can help slow down changes in water quality.

Can I go swimming when the water is green?

When the water is green or cloudy, swimming is not recommended. This means that your pool water is polluted and full of bacteria. Ears, nose and eyes can become irritated or infected (ear infections, conjunctivitis), and this can trigger skin diseases.

How do you restore crystal-clear pool water?

As soon as the water in your pool starts to turn green, don't delay! The first step is to stop swimming until the water is healthy and clean again. The longer you wait, the more you'll need expensive chemical treatments that will deprive you of swimming.

Chemical treatments for green water

Green water with green algae

If you notice that the walls are slippery or even slimy, algae growth has already begun. In this case, flocculant, which is an anti-algae agent, will enable you to eradicate algae. This product improves filter performance and the effectiveness of treatment and disinfection products. However, this product must not be used with diatomaceous earth filters, and cartridge filters require a specific flocculant. Finally, remember to check that your usual maintenance products do not already contain flocculant, as some of them do.

  • Maintain a pH value between 7 and 7.4Basic water is a favorite playground for algae. They don't like water that's too acidic. However, the higher the pH (8 and above), the more likely they are to settle in your pool.
  • Check the chlorine level, qui agit sur la désinfection de l’eau et lutte notamment contre l’apparition des algues. Faites un traitement-choc au chlore si nécessaire. Si vous utilisez un stabilised chlorine (avec du stabilisant), il est conseillé de maintenir son taux entre 0,7 et 1,2 ppm. Si vous utilisez un unstabilized chlorineit is advisable to maintain a level between 0.4 and 0.7 ppm.
  • Activate filtration and clean your filter 1x/week
  • Anti-algae (flocculant) : complete the chlorine shock treatment with an algicide if the water is particularly green and there is a lot of algae.
  • Clean your pool walls and bottom. Use a brush and a broom to scrub the walls (bottoms, water line, stairs,...) to remove the sticky marks.

Green water without algae

It's possible that your pool water is turning crystal green, with no algae. This problem can arise after adding a lot of disinfectant, or from water that's too acidic, or too low in TH.

  • Measure the pH of your pool. If it's below 7, add pH+ and filter the water.
  • Control the TAC (water alkalinity) . The TAC should be between 100 and 200 mg/litre, depending on the treatment used in your pool or spa. At the start of the season, it should ideally be between 100 and 150 mg/l.
  • Measure TH (water hardness), which should be between 100 and 200 mg/l or ppm. Here is how to rectify water hardness
  • Set up continuous filtration 
  • Use shock treatment  

Finally, if you use a TDS sensor in your salt water chlorinator pool, you don't need to measure TAC or TH. Simply check TDS measurements. Find out more about TH, TAC and TDS by clicking on the image to access our video.

Protect your pool water from the sun's UV rays

Start by checking the pH of the water every day to measure its balance. If the pH is too far outside the required range, it may be necessary to apply shock chlorine to restore clarity, or simply add a pH-correcting product (pH+ or pH-). Once the product has been added, activate filtration, respecting the following rule: water temperature divided by two = filtration time. If your pool water is still not perfect after filtration, you can use a flocculant while continuing to filter the water.

If your pool is very exposed to the sun, this problem is likely to recur throughout the season. That's why it's important to protect the water as much as possible with umbrellas and shade sails, to prevent its quality from deteriorating rapidly. This will also slow down the bacteria growth process.

What treatment to use after a rain or thunderstorm ?

The first thing to do after a heavy downpour is to measure and correct the pH of the water, which is likely to be below 7 (due to the acidity of rainwater).Secondly measure the alkalinity (TAC) of the water as well as the ORP, which indicates the level of disinfection of the water. Last but not least, there's a pool water analyzer that measures the water's disinfecting power on a daily basis.

What to do if the water remains green after treatment?

When your pool water was green or cloudy, you carried out a shock treatment that clarified the water. But it took very little time before the pool water turned green again. This may be because the pH has been over-stabilized. This means it no longer has any disinfectant effect, making the shock treatment ineffective. Unfortunately, the only thing to do in this case is to drain 1/3 of the pool water to renew it. Pour avoir la confirmation d’une sur-stabilisation de l’eau, vous pouvez réaliser un test colorimétrique de stabilisant. A normal stabilizer level is between 25 and 50 ppm (or mg/l). If the test you have carried out indicates a level above this threshold, you have no choice but to renew part of your pool water.

What to remember to treat your pool water and avoid a green pool

Now that you know what triggers a greenish pool, you can prevent the phenomenon. In high season, proper filtration and the creation of shade (sunshades) help to lower the water temperature. Water control and regular pool maintenance are essential.Don't forget that regular pool maintenance reduces the risk of water changes. Follow our tips to care of your pool step by step to maintain optimum water quality all season long. Swimming all summer long!