Green water in a swimming pool is one of the most common problems during the swimming season. It usually occurs when there is an imbalance in the water treatment, insufficient filtration or a build-up of algae.
Although at first glance it may seem to be merely a cosmetic issue, a green swimming pool is a sign that the water is not in optimal condition. Before swimming, it is advisable to test the water, adjust its parameters and restore its clarity.
For swimming pool professionals, maintenance technicians, installers and spare parts retailers, knowing how to identify the cause is key to providing a quick solution and preventing the problem from recurring.
Why does the water in the swimming pool turn green?
The colour green is often associated with the presence of algae. These algae appear when the water is no longer properly disinfected, when the pH is out of balance, or when the filtration system is not working properly.
The most common causes are:
| Reason | What causes it | Standard solution |
|---|---|---|
| Lack of chlorine or disinfectant | Algae are multiplying | Analysing and adjusting disinfection |
| Imbalanced pH | Chlorine loses its effectiveness | Adjust pH |
| Inadequate filtration | The water isn’t being replaced properly | Check the pump, filter and filtration hours |
| Dirty filter | System performance is slow | Wash, clean or replace the filter media |
| Excess organic matter | Rapid chlorine consumption | Cleaning, brushing and deep cleaning |
| Intense heat or strong sunlight | Increased algal growth | Improve monitoring and maintenance |
The CDC It points out that chlorine and pH are the first line of defence against germs in domestic swimming pools and spas, and recommends testing and adjusting both parameters on a regular basis to keep the water safe
Can you go for a swim if the pool water is green?
It is not advisable to swim in a swimming pool with green water until the water returns to the correct parameters and clarity.
Although the green colour is often caused by algae, it can also indicate insufficient disinfection or poor filtration. The CDC warns that swimming pools with correct levels of chlorine or bromine and pH are less likely to spread germs, and also recommends not swallowing water and avoiding swimming when the water is not in good condition.
For a professional, the message to the client must be clear:
First, the water is analysed and treated. Then its clarity is restored. And only when the water parameters are correct is the swimming pool ready for use again.
Step 1: Test the water before applying any products
The first mistake you make when a swimming pool has turned green is to add chemicals without testing the water.
Before taking any action, you must check:
- pH.
- Free chlorine or available disinfectant.
- Alkalinity.
- Stabiliser, if required.
- Salt level, if the pool has a salt chlorinator.
- Visual condition of the water.
- Filter pressure.
- How the pump works.
On Trypool, the category of Water treatment It includes specific products for analysis, dosing, salt chlorination and water monitoring, enabling professionals to work with a comprehensive solution, rather than just a one-off correction.
Step 2: Adjust the pH of the swimming pool
pH is one of the most important factors. If it is too high or too low, the treatment may become less effective.
The CDC recommends maintaining the pH within a range of 7.0 to 7.8. It also states that, as the pH rises, chlorine’s ability to kill germs decreases, particularly if it is above 8.0.
In both professional and domestic settings, many installations typically operate within a range close to 7,2-7,6, but you must always follow the instructions provided by the manufacturer of the product being used and the relevant regulations in each case.
Step 3: Scrub the walls, floor and areas with algae
When the water is green, the algae aren’t just floating around. They’re often clinging to the walls, corners, steps, lights, skimmers and the bottom.
For this reason, before or during treatment, it is advisable to brush:
- Walls.
- Waterline.
- Stairs.
- Areas with little traffic.
- The bottom of the swimming pool.
- Around nozzles, skimmers and spotlights.
This step helps to dislodge the algae so that the filtration system and chemical treatment can work more effectively.
Step 4: Apply an intensive treatment
When the water turns green due to the presence of algae, it is usually necessary to carry out a shock treatment or intensive disinfection.
This treatment must always be carried out:
- With the pH already adjusted.
- Follow the dosage recommended by the manufacturer.
- No swimmers in the pool.
- With the treatment plant in operation.
- Wear gloves, goggles and protective equipment if required by the product.
- Do not mix chemicals together.
The CDC It is recommended that you always read and follow the instructions on the labels, and wear protective equipment such as gloves or goggles when handling disinfectant chemicals within your water treatment category.
Common mistakes when treating a green swimming pool
– Add chlorine without checking the pH
If the pH is incorrect, the treatment may not work properly.
– Shutting down the treatment plant too early
The treatment requires filtration to remove particles and restore clarity.
– Do not scrub the walls and floor
Algae that have attached themselves can survive and multiply again.
– Using incompatible products
Not all products can be combined. Always follow the instructions on the label.
– Not checking the filter
If the filter is dirty, broken or the wrong size, the water may turn green again.
– Thinking that the salt-based chlorinator does everything by itself
A salt chlorinator helps to automate disinfection, but it requires maintenance, salt checks, cell cleaning, pH monitoring and proper filtration.
Swimming pool water often turns green due to the presence of algae, a lack of chlorine, an unbalanced pH, insufficient filtration or a build-up of organic matter.
The first step is to test the water. You need to measure the pH, free chlorine and alkalinity, and check that the filtration system is working properly before adding any products.
Chlorine can help to remove algae, but it must be applied once the pH has been adjusted and accompanied by brushing, filtration and cleaning the pool floor.
This may be due to high pH, bound chlorine, poor filtration, excess stabiliser, a dirty filter or high levels of organic matter.
Yes. A salt-water pool can turn green if the chlorinator isn’t producing enough chlorine, if there isn’t enough salt, if the cell is dirty, if the pH is too high, or if the filtration system isn’t working properly.
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At Trypool, we provide professional solutions to restore and maintain swimming pool water in optimal condition: water analysis, salt chlorinators, dosing systems, pumps, filters, filter spare parts and cleaning supplies.
If you’re a swimming pool professional, maintenance technician, installer or specialist retailer, take a look at our range of products and choose the right solution for every installation.
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