How to clear a green pool without draining it

Why does draining almost never need to happen?

Because algae is not attached to the water. It is a living thing suspended in the water and stuck to the walls, and taking the water away leaves the walls exactly as green as they were. Owners who drain a green pool usually end up scrubbing a dry green shell in the sun, which is worse work at a worse time.

Draining also throws away everything you already paid for. Your calcium hardness, your stabilizer, your alkalinity and roughly twenty thousand gallons of water you bought once. Refilling here means refilling with hard North Texas fill water, so you trade an algae problem for a scale problem.

There are real reasons to drain, and they are chemistry reasons rather than color reasons. Stabilizer far too high to chlorinate against is the usual one. That case has its own guide on this site, and it is a smaller group of pools than the internet suggests.

What has to happen before you add any chemical?

Get the big material out. Net the leaves, empty the baskets, and if there is a layer of sludge on the floor, scoop or vacuum the heavy stuff to waste rather than through the filter. Every pound of debris left in the pool is chlorine you will buy and burn on nothing.

Then brush. All of it, including the steps, the shaded wall, behind the ladder, the tanning ledge and the corners. Brushing lifts algae off the surfaces and puts it in the water column where chlorine and the filter can reach it. A pool that is chemically shocked but never brushed clears slowly and re blooms in the same three spots.

Then get the equipment honest. Clean the filter before you start rather than after, because a loaded filter will not clear a green pool. Confirm the pump runs at a speed that actually turns the water over, not the overnight energy saver speed. Confirm the returns are open and the water is rotating.

Then test cyanuric acid. This is the number that decides your whole treatment, and skipping it is why people dump chlorine into a pool for a week with nothing to show for it.

How much chlorine, and why does stabilizer decide it?

Cyanuric acid holds chlorine in reserve and protects it from sunlight, which also means it slows chlorine down. The higher your stabilizer, the higher the free chlorine reading you need for the same killing power. A pool at 30 ppm cyanuric acid and a pool at 90 ppm need very different numbers on the test, and the second one needs a lot more chlorine to do the same job.

Practically, this means you test stabilizer once, set a target free chlorine level appropriate to it, and then hold that level. Holding is the part people miss. Algae consumes chlorine, so the level you set in the morning is gone by afternoon on day one. A green pool recovery is dosing three or four times a day, not once.

Use liquid chlorine, sodium hypochlorite. Trichlor tablets and dichlor shock both add more stabilizer while you work, which raises the target you are chasing. Calcium hypochlorite adds calcium hardness, which in this region you already have plenty of. Liquid adds chlorine and salt and nothing you will regret.

Set pH to the low end of normal, around 7.2, before you start. Chlorine is meaningfully more active at lower pH, and a green pool at pH 8.0 is a pool where a good chunk of your chlorine is sitting in the weak form doing very little.

How do you know when the algae is actually dead?

Color is not the test. A pool goes from green to gray cloudy when the algae dies, and gray cloudy is a success rather than a setback. The dead cells are still in the water and the filter has to remove them.

The real test is overnight chlorine loss. Take a free chlorine reading after sunset, take another before sunrise, and compare. If the pool lost roughly a point or less overnight with no sunlight involved, nothing is left in there eating chlorine and you are done killing. If it lost several points, something is still alive and you hold the level another day.

The second check is combined chlorine. If combined chlorine is above about 0.5 ppm, the pool is still working through waste. It should fall to near zero by the end of a proper recovery.

Once both checks pass, stop shocking. From there it is pure filtration, and adding more chlorine will only slow the water down.

Why does the filter decide how long this takes?

Because after the kill, everything you were fighting is a particle, and particles leave the pool one way. Through the filter. A recovery moves at the speed of your filtration and nothing else.

This is why a green to clean involves multiple filter cleanings rather than one. A cartridge that would normally go months can load solid in six hours during recovery. When the pressure comes up eight to ten psi above the clean baseline, the filter has effectively stopped working, and every hour you leave it there is an hour of no progress.

Run the pump continuously. Not a schedule, not eco mode, continuously and at a speed that moves real water. Variable speed pumps programmed for quiet maintenance flow will keep a recovering pool cloudy for a week while the owner wonders what is wrong with the chemicals.

If the water is chemically dead and the filter is clean and it still will not clear after two full days of continuous running, a clarifier is a reasonable next step. It clumps fine particles into pieces the filter can catch. A flocculant is the heavier hammer and it drops everything to the floor for vacuuming to waste, which costs you water.

How long should the whole thing take?

A light green pool caught early clears in two to three days with disciplined dosing and continuous filtration. A dark green pool with a visible floor of sludge runs four to seven and needs several filter cleans on the way.

What extends it is almost always one of four things. Stabilizer too high to reach an effective chlorine level. A filter nobody cleaned mid job. A pump running on a light schedule. Or intermittent dosing where the level drops to zero every night and the algae gets a free shift to regrow.

Professional green to clean recovery runs $450 to $1,250+ and that band is wide for exactly these reasons. What you are buying is somebody returning every day with a truck full of liquid chlorine, cleaning the filter as often as it needs, and knowing when to stop.