How do you lower CYA in a pool?
Drain and refill: bringing stabilizer from 100 ppm down to 50 ppm means replacing 50% of the water, which in a 20,000 gallon pool is 10,000 gallons of fresh water.
Cyanuric acid barely breaks down in a chlorinated pool and nothing you pour in removes it. It leaves with the water that leaves, so lowering it is a plumbing job rather than a chemistry one.
Why a high reading costs you
The shock level is about 40% of your CYA, so a pool at 100 ppm needs 40 ppm of free chlorine to kill algae where one at 50 needs 20. Daily chlorine has to rise with it. The usual first sign is chlorine that tests fine while the water turns dull or green.
One big drain beats several small ones
Two drains of 25% each take a pool from 100 ppm to 56 ppm, not 50, because the second drain removes water that has already been diluted. Drain the full share in one go if the pool can take it, and refill straight away. Vinyl liners and pools on a high water table are the exception: drain those in stages and accept the slower result, since a shrunken liner or a lifted shell costs far more than the extra water. Test your fill water too; some municipal supplies carry CYA of their own.
What does not work
CYA reducers sold in stores are enzyme or bacterial products, and independent tests show they rarely move the number. Shock, acid and longer filter runs do nothing to it either. Mobile reverse-osmosis services, which filter the water on site, lower CYA and calcium without a drain and are worth pricing where water is expensive or draining is unsafe.
Stop it climbing back
A sunny pool burning 3 ppm of chlorine a day gains about 54 ppm of CYA a month if all of that comes from trichlor tablets, which is how most pools reach 100. Dichlor shock adds even more per dose. Once you are back at target, switch to liquid chlorine, which adds none, and keep tablets for weeks away.
Work out your own numbers
Use the CYA calculator, pool shock calculator and the chlorine dosing 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? Why is my pool still green after shocking? 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.