Green Pool? How to Fix It

Why pools actually turn green

Green water is algae — a bloom that takes hold whenever chlorine drops too low for too long, usually helped along by warm weather, strong sunlight, and something feeding it. Algae spores are in the air and land in every pool constantly; what stops them taking hold is a sanitiser level that's actually doing its job. Once chlorine falls behind — heavy rain diluting it, a hot week burning through it faster than usual, or a chlorinator that's been quietly underperforming — algae gets the opening it needs and can turn a pool visibly green within a day or two.

Phosphates make it worse. Fertiliser runoff, decaying leaves, and some tap water sources add phosphates to the water, which act as a food source that lets algae grow faster once it starts. A pool can have technically adequate chlorine and still struggle with algae if phosphate levels are high — worth knowing if a pool keeps going green despite seemingly normal sanitiser readings.

The types you'll actually run into

Green algae

By far the most common. Free-floating, turns the whole pool a cloudy green, and responds well to the standard treatment below.

Mustard algae

A yellowish, often patchy growth that clings to walls, particularly in shaded areas. More resistant to standard doses and often needs a stronger, more targeted treatment.

Black algae

Dark spots with a tough outer layer that resists chlorine. Usually needs the spots physically broken open by brushing before chemical treatment can reach it — often worth professional help for a bad case.

The fix, in the right order

1. Test and rebalance first

   Test pH and alkalinity before adding anything. Chlorine works far less effectively in unbalanced water, so shocking first is wasted effort — the same principle covered in our cloudy water guide.

2. Brush the walls and floor

   Brush every surface to physically break up algae before treating. This exposes it to chlorine and speeds up the whole process significantly — skipping this step is the most common reason a shock treatment seems to underperform.

3. Shock the pool

   Use a stronger dose than a routine shock — algae blooms typically need double or triple the normal amount to fully knock out, not a standard maintenance dose.

4. Run the filter continuously

   Run the filter around the clock until the water clears, backwashing or cleaning it partway through if pressure climbs quickly — a heavy algae bloom loads up a filter faster than normal use.

5. Vacuum dead algae to waste

   Once algae settles to the floor as a fine dust, vacuum it out on a "waste" setting if your filter has one, rather than through the filter — vacuuming that much fine debris through the filter just clogs it fast for no benefit.

6. Retest and repeat if needed

   If the water is still green after 24 hours, retest and shock again. A severe or long-standing bloom sometimes genuinely needs two full treatment cycles — that's normal, not a sign something's being done wrong.

Stopping it from coming straight back

Most repeat green-outs trace back to the same root cause: sanitiser dropping too low for too long, usually from irregular monitoring rather than any single big mistake. Testing consistently (a couple of times a week in swim season, more after heavy rain or a hot spell) and brushing regularly — even when the pool looks fine — catches the early stages of a bloom long before it's visible. An algaecide used as a preventative booster after a full clear-up can help, but it's a supplement to consistent chlorine levels, not a substitute for them.

When to call a professional

Black algae that keeps returning after proper treatment, a bloom that won't clear after two full shock cycles, or algae combined with equipment problems (see our pump and filter troubleshooting guide) are all worth a professional look rather than repeated DIY attempts — sometimes the underlying cause is a circulation or filtration issue that chemicals alone won't fix. If what you're looking at is dark, spotty, and resisting normal treatment, it may not be green algae at all — see our black algae vs green algae guide for how to tell the difference and treat it correctly.

Common questions