Let's get straight to it: green water always means algae. It's not a problem of visible dirt or dust, it's microorganisms that, under normal conditions, chlorine eliminates before they become noticeable. The water turns green when that defense stops working.
From green to clear, in order
Resolves in 24-48 hours
- A single shock treatment is usually enough
- Usually no need to drain anything
Can take several days
- Usually needs more than one round of shock treatment
- Additional brushing of walls and floor
Why algae appear
Algae are present in every pool in minimal amounts at all times, they arrive with rain, wind, swimmers and even the cleaning equipment itself. With chlorine in its normal range, they never become noticeable. The problem appears when several factors come together at once:
- Free chlorine has dropped too low, often without anyone noticing, an empty dosing unit, several days without a check, or higher consumption than usual because of more swimmers than normal.
- pH is high, which greatly reduces chlorine's disinfecting power even when its level looks correct. We have a specific guide on how to correct pH.
- High temperature and lots of sun, the conditions in which algae reproduce fastest.
- Recent rain, which dilutes the chlorine and also washes new spores into the water.
- Insufficient filtration, either because the filter hasn't been running long enough each day or because it needs cleaning or a sand replacement.
What to do the moment you see it
The general protocol followed in the industry, without going into exact product amounts (that depends on your pool's volume and how advanced the problem is, and should be calculated by whoever is going to apply it):
- First correct the pH to the right range, without this, any chlorine added afterward performs much worse.
- Apply a shock chlorination, well above the usual maintenance dose.
- Thoroughly brush walls, floor and corners to break up the layer of adhered algae so the chlorine can act on it.
- Keep the filtration running continuously for at least 24 hours, and backwash the filter the following day.
How long it takes to clear up
It depends on how advanced the problem is. A light green that has just appeared usually resolves in 24-48 hours with a single treatment. A dark green that's been established for days may need several days and more than one round of shock treatment before the water is crystal-clear again.
Do you have to drain the pool?
Almost never. Draining the water is, almost always, the last resort, it only makes sense if the chemical treatment fails to resolve it or if there's an underlying structural problem. In the vast majority of cases, the water recovers perfectly without dumping a single liter.
Frequently asked questions
Is it dangerous to swim in green water?
You shouldn't swim while it's green. Besides the lack of visibility of the bottom, it means the disinfectant isn't working and the water isn't in hygienic condition.
How long does a pool with green water take to recover?
A recent light green can be resolved in 24-48 hours; a dark, established green may need several days and more than one treatment.
Do you have to drain the pool if the water is green?
Almost never. It's almost always the last resort, only if the chemical treatment doesn't work or there's a structural problem.