How to Clear Green Pool Water (Step by Step)
Updated 2026-05-21
Green pool water means one thing: algae. The shade of green tells you how serious it is. Light green or yellow is early algae and clears in 2 days. Dark green or black green has established and needs the full 3-day shock and brush protocol. Either way, you almost never need to drain the pool.
Day 1: massive shock and brush
Algae lives on chlorine in the same way you live on food. The way to kill it is to overwhelm the colony with chlorine faster than it can consume.
Shock at triple the normal rate. For Cal-Hypo, that is 3 pounds per 10,000 gallons. Spread it around the pool perimeter at dusk so sunlight does not burn it off. Then brush every surface aggressively, especially walls and steps where algae attaches.
Run the pump 24 hours straight.
Day 2: test and re-shock if needed
Free chlorine should still be elevated (above 5 ppm) after a successful shock. If it is back to normal, the algae consumed it. Re-shock at the same rate and brush again.
Vacuum the pool floor. Dead algae settles as a fine gray dust that will resuspend if you only brush.
Day 3: filter and balance
By day 3 the water should be cloudy gray-white instead of green. That is dead algae being filtered out. Keep the pump running. Clean or backwash the filter often (you will be cleaning more debris than usual).
Once water is clear, retest everything: free chlorine, pH, alkalinity, CYA, and calcium hardness. Bring all parameters into target range.
Preventing the next bloom
Algae returns when chlorine drops or stabilizer (CYA) is wrong. Keep free chlorine between 1 and 3 ppm year round, and CYA between 30 and 50 ppm. Test weekly during pool season and after every storm.
Brush the walls weekly even when water looks clean. Algae starts as invisible film before it goes visible green.
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Do I need to drain a green pool?
Almost never. A 3-day shock and brush protocol clears 95 percent of green pools without draining. Draining is expensive and stresses the pool shell.
Why is my pool still green after shocking?
Either the shock dose was too low, CYA is too high (above 80 ppm blocks chlorine effectiveness), or you did not brush. All three are common reasons a single shock fails.
Can I swim in a green pool?
No. Algae itself is not harmful, but algae blooms harbor bacteria that can cause skin and eye irritation. Wait until water is clear and chlorine is back in range.