In summer, it’s not uncommon for swimming pool water to turn green. Why does the water turn green? How can you prevent the water from turning green? How do you treat green water? What should you do if the water turns green again? In this article, we’ll answer all these questions. You’ll find practical advice on how to get into the right habits when it comes to water testing, filtration and pool maintenance.
If you’re wondering what’s turning your pool water green, bear in mind that there are various possible causes. These may include weather conditions, technical issues with your equipment, or even human factors (natural contamination from swimmers). It is also worth noting that a high concentration of insects – or larvae – can alter the colour of the water when they are present in large numbers.
According to the Federation of Swimming Pool and Spa Professionals, faulty filtration—caused by a pump problem or clogged filters—is responsible for 80% of cases where the water turns green.
De plus, une durée de filtration trop courte modifie également l’équilibre de l’eau qui deviendra verte avant de développer des algues. C’est pourquoi il est important de savoir combien de temps il faut filtrer l’eau de votre piscine !
Leaves, sand, pollen... the water in your swimming pool is full of dirt, especially if it's outdoors! So you need to clean it regularly.
During a heatwave, the water in your pool heats up and does not cool down overnight. As soon as the water temperature exceeds 28°C, bacteria begin to multiply. The water then becomes cloudy and turns green. The pool walls become slippery. Unless you act quickly, green algae will appear. You must take action as soon as the water becomes cloudy!
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.
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.
After a heavy downpour, it is necessary to measure :
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.
When you restart your pool after wintering, it's very common for the water to have turned green, so there's nothing to worry about. In this context, a restarting your pool should be enough to solve the problem.
pH is the first line of defence against algae growth. A pH level between 7.2 and 7.4 ensures that the chlorine remains effective and that algae find an environment that is not conducive to their growth. It is advisable to measure it at least two or three times a week during the peak season, and after any event that might affect it: heavy rain, heavy swimming, or high temperatures.
The free chlorine level – or, better still, the ORP – must also be monitored regularly. A lapse of just a few hours is enough to give algae free rein.
If you don’t take regular, accurate measurements (pH, disinfection and temperature), you risk adding chemicals unnecessarily or too late. A connected water analyser provides these readings at the right time, especially during hot weather.
Everything you need to know about pool water quality parameters to watch out for
Finally, you should know that there is ICO Pool, a smart water analyser for swimming pools that measures the pH, temperature and disinfection levels of the water.
Filtration is at the heart of the treatment system. Poorly circulated water stagnates, warms up and creates dead zones that are conducive to algae growth. In summer, the recommended filtration time is half the water temperature, i.e. around 13 hours for water at 26 °C.
During hot weather or after swimming a lot, don’t hesitate to extend this time.
Also ensure that the discharge nozzles are correctly positioned to create an even flow throughout the tank.
Everything you need to know about swimming pool filtration
Algae tend to attach themselves to walls, corners and steps.
Brushing surfaces once a week, followed by a quick hoover, removes deposits before they turn into visible colonies.
The skimmer basket should be emptied several times a week: a blocked skimmer becomes less effective and allows organic debris to decompose in the water.
The filter itself must be flushed or backwashed as soon as the pressure exceeds the reference value by 0.3 to 0.5 bar.
Follow us advices regular maintenance of your pool step by step pour conserver une qualité d’eau optimale toute la saison.
A preventive algaecide treatment, distinct from chlorine, creates a chemical barrier against algae spores before they have time to take hold
It is recommended that you apply:
Be sure to choose an algaecide that is compatible with your treatment system and to follow the recommended dosages.
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.
| aspect | actions to be taken | |
| green water with algae | slippery, or even sticky, surfaces | – check the pH and ORP – add: flocculant intensive treatment if necessary algaecide if necessary – filtration – cleaning |
| green water without algae | crystal-clear green water with no algae | – check pH, total acidity (TAC) and total alkalinity (TH) – continuous filtration – intensive treatment if necessary |
| the water remains green after treatment | – check the stabiliser |
If you notice that the walls are slippery or even slimy, algae growth has already begun.
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.
When the problem recurs quickly, it is often a sign that the water balance is not being monitored regularly or closely enough (temperature fluctuations, inadequate disinfection, etc.). With a connected water analyser such as ICO, continuous monitoring combined with alerts means you don’t have to wait for the water quality to deteriorate.
Your pool water was green or cloudy, so you added flocculant and carried out a shock treatment. But the water turned green again very quickly. It may be that your pool water is over-stabilised, which is making the shock treatment ineffective.
To confirm whether your water is over-stabilised, you can use a test strip to measure cyanuric acid (stabiliser). An acceptable level of stabiliser 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 option but to empty 1/3 and renew the water in your pool.
Now that you know what causes a greenish pool, you’ll be able to prevent this from happening. During the peak season, effective filtration, regular water testing and routine pool maintenance are essential. Regular water testing can be done manually (using test strips or electronic testers) or automatically (using a connected analyser).
It's all yours for swimming all summer!
When the water is green or cloudy, it's not recommended to swim. This means that your pool water is polluted and full of bacteria. Your ears, nose, and eyes might get irritated or infected (ear infections, conjunctivitis) and it can trigger skin diseases.
– If the water is green with algae present: check pH and disinfect, add flocculant, shock treatment and algaecide if necessary
– If the water is green and crystal clear, without algae: check pH, total alkalinity, total hardness, shock treatment if necessary
– Check the stabilizer level: if it is above 50 ppm, replace some of the water in your pool
With several combined actions (shock chlorine + flocculant + 24-hour filtration), pool water generally becomes clear again in 24 to 48 hours. For water heavily laden with algae, it can take up to 72 hours.