How to Clean a Green Pool: Maintenance, Chemicals, Filtration and a Robotic Pool Cleaner

The most important lesson with a swimming pool is that water does not forgive long delays. If something goes wrong, it is better to react in time. The longer green water sits, the more work comes afterwards — more filtration, more cleaning, more chemicals and more nerves.

This is not a laboratory manual, but practical experience: what I check first, how I approach a green pool, what I use for cleaning and what I would recommend to someone who is just starting to maintain a swimming pool in the garden.

Why a Pool Turns Green

Most often, the green colour comes from algae growth. Algae appear when the conditions become suitable for them — warm water, strong sun, insufficient disinfection, incorrect pH, weak circulation or organic debris in the water.

The cause may be one thing, or a combination of several:

• low chlorine or low level of another disinfectant;
• incorrect pH;
• insufficient filtration time;
• a dirty filter;
• too many leaves, dust or organic materials in the water;
• strong heat;
• rain and wind;
• interrupted pump operation;
• poor circulation;
• missed regular maintenance.

In our case, the riskiest moments are hot days and the periods after rain or strong wind. Then more dust, leaves and organic material enter the water, and the pool loses balance more quickly.

First, Check the Cause

When a pool turns green, the first instinct is often to add a product and wait for a miracle. However, this rarely works well if we do not know what is happening in the water.

The first thing to check is pH. If the pH is not within the right range, chemicals do not work effectively. You may add chlorine, but it will not perform as it should if the water is not balanced.

With a green pool, I start with three things:

• checking the pH;
• checking the chlorine level or the disinfectant being used;
• checking whether the pump and filtration system are working properly.

Only after that does it make sense to think about shock treatment, cleaning and clarifying the water.

Step One: Remove the Large Debris

Before adding any chemicals, it is best to remove everything visible: leaves, twigs, insects, mud, dust and fallen flowers. The more organic material there is in the water, the more pressure it puts on the chemicals and filtration system.

I use a surface net, and if there is a lot of dirt on the bottom, I do not wait for the filter to collect it all. First, the pool needs to be mechanically cleared of the larger debris.

Step Two: Adjust the pH

pH is the foundation. If it is not within the correct range, the rest of the treatment becomes less effective.

For pool maintenance, it is always useful to have a tester — liquid, tablet, strip or electronic. There is no need to guess by the colour of the water. The water needs to be measured.

If the pH is too high or too low, correct it first according to the instructions of the product you are using. The dosage depends on the pool volume and the specific product.

Step Three: Shock Treatment

A green pool usually needs shock treatment. This means a higher dose of disinfectant, used to destroy algae and microorganisms.

This should not be done “by eye”. The dose needs to be calculated according to the volume of the pool and the instructions of the specific product.

After shock treatment, the pump needs to run long enough to circulate the entire volume of water. With heavily green water, one short cycle is often not enough. Many hours of filtration may be needed, and sometimes the treatment has to be repeated.

Filtration Needs to Run for a Long Time

When the water turns green, filtration is just as important as the chemicals. The chemicals destroy the algae, but the filter has to remove the dead organic matter and fine particles from the water.

With green water, the pump needs to run longer than usual. If you have a sand filter, backwashing may be necessary. If you have a cartridge filter, clean it or replace it depending on its condition.

Very often, people add chemicals but do not leave the filtration running long enough. As a result, the water is no longer bright green, but it remains cloudy.

Cleaning the Bottom, Walls and Waterline

Green water almost always leaves marks on the walls, bottom and waterline. Algae like surfaces. If you treat only the water but do not clean the walls, the problem can return.

This is where a robotic pool cleaner becomes a big help. I use a Zodiac Alpha RA6900iQ robotic cleaner. For me, the most important thing is that it cleans not only the bottom, but also the walls and the waterline. The waterline is exactly where grease, dust, stuck particles and early dirt often collect.

The robot does not replace water treatment. If the pool is green, the water needs to be corrected first — pH, shock treatment and filtration. The robot helps remove what has settled on the bottom, walls and waterline.

How I Use the Robot in Practice

For normal maintenance, I run it when there is dust, sand, leaves or dirt on the bottom, especially after wind. After heavier contamination, it is better to remove the larger debris first with a net, so the robot filter does not fill up too quickly.

After every cleaning cycle, the robot filter needs to be removed and washed. This should not be postponed. If the filter stays full of dirt, the robot works less efficiently and loses performance.

For me, the robotic cleaner is one of those purchases that really changes pool maintenance. Not because the pool suddenly maintains itself, but because it saves the most annoying physical work.

My Order for Cleaning a Green Pool

1. I remove leaves, twigs and visible debris with a net.
2. I check the pH and correct it if needed.
3. I check the filter and pump.
4. I do a shock treatment according to the pool volume and the product instructions.
5. I leave the filtration running for a long time.
6. I clean the bottom, walls and waterline with the robotic cleaner.
7. I wash the robot filter after the cycle.
8. I backwash or clean the main filter if necessary.
9. I check the pH and disinfectant level again.
10. I repeat filtration and cleaning until the water clears.

How to Avoid Getting a Green Pool

The best maintenance is the kind that prevents the water from turning in the first place.

• check pH regularly;
• monitor the chlorine level or the disinfectant you use;
• provide enough filtration time;
• remove leaves and organic debris in time;
• run the robotic cleaner when needed;
• clean the filter;
• check the water after rain, wind and very hot days;
• use a pool cover if you have a suitable one.

With our pool, the most important things are consistency and reacting in time. If the water starts to lose clarity even slightly, I do not wait for it to turn green. I check the pH, filtration and visible dirt. This makes the problem much easier to solve.