A green pool almost always means algae, triggered by low chlorine, high pH, poor circulation, or a clogged filter. Fix it by brushing every surface, balancing pH to 7.2, shocking with 3–4 doses of liquid chlorine, and running the pump 24 hours. Clear water usually returns in 48–72 hours.
What green water actually is
Green pool water is algae. Specifically, it's a bloom of free-floating single-celled algae — usually green algae (Chlorophyta), sometimes mustard algae, rarely black algae. All three feed on sunlight, phosphates, and the organic matter that ends up in every outdoor pool. Chlorine is what normally keeps them in check. When chlorine falls below roughly 1 ppm — even for a day or two — a bloom can start, and by day three you can see it.
The four common causes
- Chlorine dropped too low. A thunderstorm, a hot week with heavy swimmer load, or a stabilizer level that's too high can all crash chlorine.
- pH is too high. Above 7.8, chlorine loses roughly half its killing power. Alkalinity and pH creep upward all summer in Westchester.
- The filter is clogged or the pump isn't running long enough. Water needs to turn over completely at least once per day. Below that, chemistry can't reach the whole pool.
- Phosphates are elevated. Lawn fertilizer runoff and organic debris introduce phosphates, which are algae food. When phosphates climb, chlorine has to work harder.
The 48-hour green-to-clean process
If your pool has turned green, work in this order. Skipping steps or reversing the order is the reason DIY green-to-clean attempts fail.
1. Test and adjust pH first
Bring pH down to 7.2. Alkalinity should sit 80–100 ppm. At the low end of pH, chlorine is dramatically more effective. Don't skip this — shocking a high-pH pool wastes most of the chemical.
2. Brush every surface
Algae attaches to walls, steps, and the tile line. Brushing lifts it into the water column where chlorine can hit it. Missing this step means the algae keeps re-seeding after you shock.
3. Shock with liquid chlorine
Use liquid chlorine (12.5% sodium hypochlorite), not powder. You want to reach 10× your combined chlorine reading — usually 3–4 gallons for a 20,000-gallon pool. Pour it slowly around the perimeter at dusk so UV doesn't burn it off before it works.
4. Run the pump 24 hours
Circulation moves chlorine through every part of the pool. Skip the timer for 24–48 hours.
5. Clean the filter
As algae dies, it clogs the filter. Backwash a sand filter every 6–8 hours during recovery, or hose out a cartridge filter twice a day.
What the pool should do over 48–72 hours
Hour 12: water turns from green to cloudy gray. This is dead algae. Hour 24: water lightens further. Hour 48: cloudy but noticeably lighter. Hour 72: clear again, though you may need a clarifier or floc for the last 20%. If you're not seeing progression by hour 24, the shock dose was too small or the filter isn't circulating. Repeat the shock and check the pump.
When to call a professional
Call for help if any of these apply: the water is opaque and you can't see the second step from the deck; you've already shocked once with no change; there's visible black or dark-green staining on the walls that doesn't brush off; or you have a party or event within a week. Our green-to-clean service in Westchester runs $450–$900 for most residential pools and includes chemistry, filter service, and daily monitoring until clear.
How to prevent the next bloom
Weekly professional service, cyanuric acid held at 30–50 ppm, a phosphate remover in June, and running the pump long enough to fully turn over the water daily. Do those four things and you won't see green water again this decade.
Related service
See our green-to-clean service