Can you over-shock a pool?
You can, but not in the way most people fear: taking a pool to 16 ppm with dichlor adds 13.5 ppm of CYA at the same time, and that — not the chlorine — is what over-shocking really costs you.
Chlorine itself has no practical ceiling in a pool. What comes with it does, and none of that leaves without draining water.
The chlorine is not the problem
Free chlorine at 20 or 30 ppm looks alarming on a test kit, but it is ordinary practice during a SLAM, where a pool is held above shock level for days until algae dies. The water is not harmed, the chlorine breaks down on its own, and the cost is the swim delay plus product you did not need.
What actually builds up
Cal-hypo adds roughly 10.5 ppm of calcium hardness for every 15 ppm of chlorine it delivers, and dichlor adds about 13.5 ppm of CYA for the same. CYA is the one that bites, because it raises the shock level for every future shock: a pool shocked with dichlor all summer needs steadily bigger doses to do the same job. Neither calcium nor CYA leaves except by draining and refilling.
Bleaching and equipment
Undissolved cal-hypo granules bleach vinyl liners wherever they settle, so dissolve them in a bucket first or broadcast them over the deep end with the pump running. Solar covers fade under sustained high chlorine, and an automatic cover closed over shock-level water traps fumes underneath. Leave the cover off overnight after shocking.
How to shock as much as you like
Liquid chlorine leaves nothing behind but a little salt, which is why it is the right product for a green pool: you can dose it daily for a week without moving calcium hardness or CYA a point. Buy it fresh and add it at dusk, so sunlight does not undo half of it.
Work out your own numbers
Use the pool shock calculator, CYA calculator and the calcium hardness 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? How many bags of shock does my pool need? Why is my pool still green after shocking? 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.