My swimming pool turned green – what should I do about It?

Few things are more inviting on a warm day than clear, sparkling swimming pool water. So discovering that your pool has suddenly developed a cloudy green appearance can come as an unwelcome surprise.

Green pool water is a relatively common problem, particularly during warmer weather, but it is also a sign that something within the pool’s water treatment system needs attention. Understanding what has caused the problem is the first step towards restoring the water - and preventing it from happening again.

Why has my swimming pool turned green?

In many cases, green pool water is caused by algae. Algae can multiply surprisingly quickly, particularly during periods of warm weather when conditions are ideal for growth.

This commonly happens when the level of sanitiser in the pool becomes too low to control algae effectively. However, water chemistry is only part of the picture. Poor circulation, inadequate filtration, heavy pool use, unusually warm weather or a problem with the pool’s dosing or filtration equipment can also contribute.

This is why simply adding more chlorine without understanding the underlying cause may not provide a lasting solution.

How do you clear a green swimming pool?

The first step is to test the pool water to establish its chlorine and pH levels and determine the extent of the problem.

Where algae growth is relatively minor, correcting the water balance and applying an appropriate shock treatment may be enough to bring it under control. Depending on the circumstances, an algaecide may also form part of the treatment.

The pool’s circulation and filtration systems then need to work effectively to remove the dead algae and other suspended particles from the water. The pool may initially appear cloudy as the algae are treated, so filtration, cleaning and further water testing are important parts of the recovery process.

A green pool should not be used until the water has been restored to the correct chemical balance and is clear enough for swimmers to see the bottom of the pool.

What if the water is very dark green?

The darker and more opaque the water becomes, the more established the algae problem is likely to be. A severely affected pool may require repeated treatment and specialist attention rather than simply adding chemicals to the water.

Importantly, a very green pool does not automatically need to be drained. The appropriate treatment will depend on the condition of the water, the type of pool and the effectiveness of its filtration and circulation systems.

For a badly affected pool, it is advisable to ask a professional swimming pool engineer to assess the water and equipment and determine the most appropriate course of action.

How can I prevent my pool from turning green?

Prevention is much easier than dealing with an established algae problem.

Maintaining the correct sanitiser level is essential, but good pool care involves more than chlorine alone. The pH and alkalinity should also be monitored, as the effectiveness of chlorine is influenced by the overall balance of the water.

Regular filtration and circulation are equally important. Filters need to be kept clean and maintained correctly, while pumps, dosing equipment and other plant-room components should be checked to ensure they are operating as intended.

During periods of particularly warm weather or heavy pool use, the water may need closer monitoring because conditions can change more quickly.

Regular servicing can prevent small problems from becoming bigger ones

A well-maintained swimming pool should provide clear, safe and inviting water throughout the swimming season. Regular professional servicing helps achieve this by monitoring water quality and checking the equipment responsible for circulation, filtration, and treatment.

An experienced service engineer familiar with your pool and plant room can often identify small changes before they develop into larger problems, helping to avoid poor water quality, unnecessary disruption, and potentially more expensive remedial work.

For more detailed information, read our guide, How to Combat Swimming Pool Algae.

London Swimming Pool Company provides professional swimming pool maintenance, servicing and refurbishment. If your pool needs regular care, water-quality advice or specialist attention, contact us for an initial discussion.