Why is my pool still green after shocking?
Most often because the dose never reached shock level: at a stabilizer reading of 60 ppm, algae only dies with free chlorine held at 24 ppm, which in a 20,000 gallon pool takes 3 gal 87 fl oz of 12.5% liquid chlorine from 1 ppm, and it has to stay there until the water clears.
One big dose on a Saturday evening kills some algae and feeds the rest. Clearing a green pool is a level held for days, checked with a test kit rather than by eye.
The dose was too small for your CYA
Stabilizer shields chlorine from the sun and from algae alike, so the kill level rises with it: 16 ppm at CYA 40, 24 at CYA 60, 32 at CYA 80. A label dose of one pound per 10,000 gallons adds about 7.8 ppm, short of all three. Test CYA before you buy anything. Above 80 or so, draining part of the water is often cheaper than chasing the shock level.
It was not held long enough
Algae burns chlorine as fast as you add it, so the level falls within hours. Retest two or three times a day and top up to the shock level each time. The pool is done when the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine drops no more than 1 ppm overnight. That usually takes three days to a week.
Green that is not algae
If the water turns green, teal or brown within minutes of shocking and stays clear rather than cloudy, it is probably metal: copper from an algaecide or ionizer, or iron from well water, which chlorine oxidizes into color. More shock will not help. Test for metals and use a sequestrant.
Cloudy is progress
Dead algae turns the water grey-blue and cloudy. Run the pump around the clock, brush the walls daily, and clean the filter whenever its pressure climbs. The last of the haze can take a few days to filter out.
Work out your own numbers
Use the pool shock calculator, CYA calculator and the pump run time calculator with your own readings. If you do not know your volume yet, measure it with the pool volume calculator.
More guides
How long after shocking can you swim? Can you over-shock a pool? How many bags of shock does my pool need? How do you lower CYA in a pool? How much chlorine do you need to open a pool? How do you raise pool alkalinity without raising pH? How much does it cost to run a pool pump? How often should you shock a pool? See all pool care guides.