What turns pool water green
The green tint you see is algae — microscopic plant life that blooms in water that isn't properly sanitized. Algae spores constantly blow into your pool on the wind, in rain, and on swimwear. As long as your free chlorine stays in the 1–3 ppm range, those spores are killed before they can multiply. When chlorine drops to zero, even for a day or two in warm, sunny weather, the spores explode into a full bloom.
A pool can go from clear to green overnight after a hot day, a big storm, or a stretch where the pump wasn't running. The most common triggers are a chlorinator that ran out of tablets, a pump or filter that was off, heavy rain that diluted the water, or cyanuric acid (stabilizer) that climbed so high it neutralized the chlorine's killing power.
Light green usually means an early bloom you can clear in a day. Dark green or murky water means a heavy bloom that will take several days and a lot more chlorine. Black or blue-green staining on walls points to stubborn black algae that needs scrubbing as well as shocking.
Step 1: Test and balance before you shock
Start by testing your water. You're checking free chlorine (likely near zero), pH, total alkalinity, and ideally cyanuric acid. Chlorine works far better in slightly acidic water, so bring pH down into the 7.2–7.4 range before shocking. If pH is above 7.6, your shock will be much less effective.
Check cyanuric acid too. If it's above about 80–100 ppm, chlorine becomes sluggish and you may need to dilute the pool with fresh water before any amount of shock will work. This single factor is why many people 'shock and shock' a green pool with no result.
Step 2: Shock the pool hard
Clearing a green pool requires raising free chlorine far above normal — this is called reaching breakpoint. For a light bloom, a standard dose of calcium hypochlorite or dichlor shock may be enough. For a dark green pool, you may need a double or triple dose. The goal is to overwhelm the algae faster than it can reproduce.
Add shock in the evening or after sunset so the sun doesn't burn it off before it can work, run the pump continuously (24 hours if possible), and brush the walls and floor to break up algae clinging to surfaces. Retest after a few hours and re-dose if chlorine isn't holding above 5 ppm.
Step 3: Filter, brush, and clear the dead algae
Once the chlorine kills the algae, the water often turns cloudy gray, blue, or milky white — that's a good sign, it means dead algae is now suspended in the water. Keep the filter running continuously and clean or backwash it as pressure rises. A clarifier or flocculant helps clump the fine dead particles so the filter can capture them.
Brush the pool daily and vacuum settled debris to waste if your filter allows. Most green pools clear within 2–5 days depending on severity. Keep free chlorine elevated until the water is fully clear and a test shows chlorine holding steady overnight.
How to keep it from coming back
Prevention is simply consistent sanitation. Keep free chlorine in the 1–3 ppm range at all times, keep cyanuric acid in the 30–50 ppm range so chlorine stays effective in sunlight, run your pump long enough each day to turn the water over, and test at least twice a week — more often in hot weather.
The fastest way to stay ahead of green water is to never let chlorine hit zero. Snap a photo of your test strip with PoolHakr and you'll know the moment your levels start sliding, before algae ever gets a foothold.
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