Green pool recovery in 5 days (2026 guide)

Day 0: assess and balance

Before you spend a dollar on chemicals, test the water. The two numbers that decide your fight are CYA and pH.

If CYA is over 60 ppm, drain a third of the water and refill before you start. You cannot win against algae if you cannot get chlorine to its shock level.

If pH is over 7.6, lower it to 7.2 with muriatic acid. Chlorine works much faster at slightly acid pH.

Clean the filter completely. A dirty filter is going to choke on dead algae in 12 hours.

Day 1: SLAM (shock level and maintain)

SLAM means raise free chlorine to 10x your CYA level and keep it there. So 30 ppm CYA = 30 ppm free chlorine target.

CYA (ppm)	Shock target FC (ppm)	Liquid chlorine for 20k gal
20	20	1.6 gal
30	30	2.4 gal
40	40	3.2 gal
50	50	4.0 gal

Add liquid chlorine in the evening. Brush every wall and the floor. Run the pump 24 hours.

Day 2: retest and recharge

Test free chlorine in the morning. It will have dropped a lot. Bring it back to the shock target. Brush. Run the pump.

The pool will go from green to gray. That is dead algae. You are winning.

Day 3 to 4: clean the filter, keep brushing

Backwash or rinse the filter every 12 to 24 hours. The water will get cloudy and milky. Keep chlorine at shock level. Keep brushing.

Day 5: the overnight chlorine loss test

When you think you are done, do this test. At sunset, record free chlorine. In the morning before sunrise, retest. If you lost less than 1 ppm and the water is clear, you are done. Let chlorine drift down to 1 to 3 ppm and resume normal care.

If you lost more than 1 ppm, hold shock another day.

What about algaecide

Polyquat 60 algaecide is a useful preventive at season opening. It is not a fix for an active green pool. Save it for after recovery.

When to call a pro

If the pool will not respond after 5 days of correct SLAM, you may have mustard algae, black algae, or a phosphate problem feeding new growth as fast as you kill it. A pool pro can identify and treat in one visit.

Frequently asked questions

Will algaecide alone clear a green pool?

No. Algaecide is a preventive. Once a pool is green, you need chlorine at shock level for several days. Algaecide added on top wastes money and can cause foaming.

How much will it cost to clear a green pool?

For a 20,000 gallon pool with moderate algae, plan on $80 to $200 in liquid chlorine and acid plus 4 to 6 hours of your time over 5 days. A service company will charge $250 to $600 to do it for you.

Can I swim before the pool is fully clear?

No. Cloudy water hides the bottom (drowning hazard) and high chlorine is rough on skin and swimsuits. Wait until clarity is back and chlorine drops below 5 ppm.

Do I need to drain a green pool?

Only if CYA is over 60 ppm and you cannot reach the shock target, or if the pool is solid swamp green and the cost of chlorine is more than the cost of refilling. Most green pools recover without draining.

My pool turned green overnight. Why?

Free chlorine hit zero. Common causes: high CYA blocking your chlorine, salt cell off or failing, hot weather plus heavy use, or a storm that diluted everything. Find the cause or it will happen again.

What is mustard algae?

Mustard algae looks like fine yellow dust on shaded walls and steps. It resists normal chlorine levels. Treat with a sustained shock for 3 to 5 days plus a mustard-specific algaecide and brush hard.

Can I save the pool without a working pump?

No. Circulation is half the battle. Fix or replace the pump first. A green pool with a dead pump is a refill, not a recovery.

Green pool recovery: from algae to crystal clear in 5 days