How do I fix a green pool?

Why pools go green

Green water is almost always algae. It grows when free chlorine drops too low, often after a spell of warm weather, heavy rain, the pump being off, or when opening the pool after winter. Once algae takes hold it uses up chlorine very quickly, so a normal dose just disappears.

The good news: even a very green pool can usually be cleared in a few days with the right steps.

Before you start

Keep everyone out of the pool until it is clear. Wear gloves and eye protection when handling chemicals, and never mix different products - add each one separately, to water, with at least an hour between them.

Step by step

• Remove leaves and debris with a net, and empty the skimmer and pump baskets.
• Test the water. Bring pH to 7.2-7.4 - shock works far better at lower pH. Wait an hour.
• Shock heavily in the evening. For a green pool you typically need two or three times a normal shock dose - the app's plan gives you a starting point for your volume.
• Dissolve the shock in buckets of pool water (chemical to water) and pour it around the edges with the pump running.
• Brush the walls, steps and floor thoroughly to break up the algae.
• Run the pump 24 hours a day. Backwash or clean the filter at least once a day while it clears - it will clog quickly.
• Test each morning. If free chlorine has dropped to zero, shock again. Keep going until chlorine holds overnight.

Green, then grey, then clear

As the algae dies the water turns grey or cloudy blue. That means it is working. A clarifier helps the filter pick up the fine dead algae; if the pool is very cloudy, a flocculant drops it to the floor so you can vacuum it to waste.

Expect three to seven days from green to clear. Swim again once the water is clear, free chlorine is back in the 1-3 ppm range and pH is balanced.

Stopping it coming back

Test two or three times a week, keep free chlorine at 1-3 ppm and stabiliser at 30-50 ppm, run the pump at least 8 hours a day and brush weekly. If stabiliser is very high (over 100 ppm), dilute it - high stabiliser is a common reason algae keeps returning.

When to call a professional

If the water turns clear green or brown after shocking, the problem may be dissolved metals such as copper or iron rather than algae. Stop shocking and get a professional water test before treating further. The same applies if the pool is so green you cannot see the floor after a week of treatment.

Frequently asked questions