The pool has turned green: what causes it and how to clear the water

Why the water turns green

Algae spores get into the pool all the time, on the wind, in rain, on leaves and on swimwear. As long as there is enough free chlorine, they die before they can multiply. Once the chlorine drops, in warm water they can turn the pool green within one or two days. The usual causes:

• a heatwave or lots of swimmers, using up chlorine faster than it was being added;
• a storm, dust or leaves, bringing in organic matter and nutrients for algae;
• pH above 7.6, so the chlorine works slowly;
• too much stabilizer, holding the chlorine back;
• a pump running too few hours, or a filter that needs backwashing;
• an empty tablet dispenser after a few days away.

If the water is clear but you see spots or a coating on the walls, see the types of algae in the pool.

Steps to recovery

1. Clear the big debris. Take out leaves with the net and empty the skimmer baskets and the pump pre-filter.
2. Adjust the pH. Bring it to around 7.2–7.4, so the chlorine you add works as well as possible.
3. Brush. Walls, floor, steps and corners, with a nylon-bristle brush. Never use a metal brush on a liner.
4. Shock chlorinate in the late afternoon, with liquid chlorine or with granules first dissolved in a bucket, in front of the return inlets. The greener the water, the bigger the dose; see shock chlorination.
5. Filter continuously. Run the pump around the clock until the water clears.
6. Clean the filter. Dead algae clog it quickly, so backwash as soon as the pressure rises, possibly every day.
7. Test and repeat. If free chlorine drops a lot overnight, the algae are still consuming it: shock again and brush again.
8. Vacuum the floor. Dead algae settle as a greyish dust. If there is a lot of it, vacuum with the filter valve set to waste, so it does not go back into the water.

The water usually goes from green to a cloudy blue and then clear. If the cloudiness persists once the chlorine has stabilized, follow the advice on cloudy water.

What not to do

• Do not empty the pool. In a liner pool, the water holds the liner in place, and green water is recovered with chemistry and filtration. See why we don't empty a liner pool.
• Do not add flocculant before the algae are dead; you will just gather live material on the floor that turns green again.
• Do not throw chlorine granules straight into the water, where they settle on the liner and bleach it.
• Do not swim until the water is clear and free chlorine is back at 1–3 mg/l.

When the water is green but clear

Water that is crystal clear but has a greenish or brownish tint, especially right after a shock or after filling with borehole water, is usually caused not by algae but by dissolved metals, such as iron or copper, oxidized by the chlorine. More chlorine does not help here; you need a metal sequestrant and, if the problem keeps coming back, a water analysis. Left untreated, the metals stain the liner.

Questions

See also