Green pool water means algae is growing in the water. Algae multiplies quickly in hot weather, in still water and when disinfection is not enough. With early action, most pools recover within a few days. But if the cause is not dealt with, the water turns green again a few days later. This guide helps you find the cause first, then bring the pool back step by step.
The most common causes of green water
- Not enough free chlorine. Chlorine is used up quickly in the sun and with heavy use. Chlorine added without testing may not be enough.
- High pH. As pH rises, chlorine's ability to kill algae drops sharply. Even if the chlorine level looks fine, it may not be working.
- Poor circulation. A pump that runs too little, blocked baskets or a dirty filter mean the water does not circulate enough.
- Too much stabiliser (cyanuric acid). With regular use of chlorine tablets it builds up over time and can suppress chlorine's effect.
- Organic load. Leaves, pollen, dust and sand feed the algae.
- A house left empty for a long time. A pool with no one watching the pump timer and chemicals can turn green within a few weeks.
Test first, then act
Use a test kit or test strips to measure three values: pH, free chlorine and total alkalinity. The ranges below are a general reference; they can vary with the type of pool and the products you use.
- pH: about 7.2–7.6
- Free chlorine: about 1–3 mg/L
- Total alkalinity: about 80–120 mg/L
The results give you a clue to the cause. If the pH is high, you need to correct it first; if chlorine is close to zero, disinfection is not being done regularly.
Step-by-step recovery
- Remove visible debris. Scoop out leaves and empty the skimmer and pump pre-filter baskets.
- Brush the walls and floor. Algae clings to surfaces. Without brushing, the chemicals cannot work properly.
- Adjust the pH. Bring the pH into the recommended range before shock chlorination. Otherwise the chlorine is wasted.
- Shock chlorinate. Apply the dose on the product label. Since the sun uses up part of the chlorine, evening is usually preferred.
- Run the filter continuously. Keep the pump running without a break until the water is clear. If the filter pressure rises, clean the filter or backwash it.
- Use an extra product if needed. An algaecide or flocculant should only be considered after the basic steps are done.
- Test again. Repeat testing and correcting until the chlorine level is stable and the water is clear.
Safety: Never mix chlorine and pH-reducing acid in the same container, and never pour them one after the other in the same spot. Store the products separately in a closed, cool place. Wear gloves and eye protection when handling chemicals, and don't swim until the treatment is finished and the values are back in range.
When to ask a specialist
- If the water keeps turning green.
- If chlorine tests high but the algae will not go. This often points to a build-up of stabiliser, and the usual solution is to replace part of the water.
- If the pump, filter or a valve is faulty, or the water level is dropping for no clear reason.
- If the water is brown, tea-coloured or very cloudy.
- If you will be away from the house for a long time and cannot keep up regular checks.
Preventing it from happening again
The most reliable way to prevent green water is regular testing and regular circulation. In summer, the aim is for the pool water to pass through the filter several times a day. How long the pump needs to run depends on the pool's volume and equipment. Reducing the leaves and dirt getting in, and cleaning the filter and baskets regularly, also make a difference.
If you don't live at the house full time, a weekly or fortnightly maintenance programme is the lowest-risk solution. The pool maintenance service covers exactly this: testing, balancing, filter checks and a record of every visit.
This guide is for general information. Every home is different; for your own home, ask a company that has seen the work on site for a written quotation.