This is the #1 cause of shock failure. If your cyanuric acid is above 80 ppm, the chlorine you add is so tightly bound that it cannot effectively kill algae. You can dump 10 lbs of shock into a pool with 150 ppm CYA and the pool will stay green because the active form of chlorine (hypochlorous acid) is barely present.
Test your CYA. If it is above 80 ppm, do a partial drain (drain 25–50%) and refill before re-shocking. The shock will not work otherwise.
At pH 8.0, only 21% of your chlorine is in the active hypochlorous acid form. The rest is the much weaker hypochlorite ion. Shocking a pool at pH 8.0 wastes 79% of your shock. Test pH and bring it to 7.2–7.4 before adding any shock chemical.
Many pool owners add one 1-lb bag to a green pool and expect results. A moderately green pool needs 2–3 lbs per 10,000 gallons. A dark green pool needs 3–4 lbs per 10,000 gallons. One pound simply is not enough to overwhelm an established algae colony.
If you shocked in the afternoon, UV sunlight may have destroyed 50%+ of the chlorine before it had a chance to kill the algae. Always shock at dusk or after dark.
Shocked the pool but did not run the pump? Dead algae cannot be filtered out, and chlorine does not circulate properly. Always run the pump for at least 8 hours after shocking — 24 hours is ideal.
If you have shocked multiple times and the pool is still green, get a full water test at a pool store including CYA, phosphates, and metals. CYA lock and high phosphates are the two most common hidden causes of treatment-resistant algae.
Still green after 24–48 hours = likely CYA, pH, or dose issue. The shock did not work.
Clear for a few days then green again = shock worked, but the underlying conditions allow algae to return. Check CYA, phosphates, filter run time, and how quickly chlorine drops between additions.
Log your CYA, pH, and shock treatments in PoolLens. The trends tell the story — find out whether you have a CYA problem or a dosing problem before you spend more on chemicals.
Open PoolLens Free →Light green: 2 lbs per 10,000 gallons. Moderately green: 3 lbs. Dark green or black: 4–5 lbs and multiple treatments over 2–3 days. Adjust pH to 7.2–7.4 before shocking for maximum effectiveness.
Yes. CYA above 80–100 ppm dramatically reduces chlorine's sanitizing power. A pool with 150 ppm CYA may have near-zero effective kill power even at 10 ppm free chlorine. The only fix is a partial drain to dilute CYA, then re-shock.
Allow 24–72 hours after shocking. The pool should become cloudy (dead algae) then clear with continuous filtration. If still green after 48 hours, re-shock at a heavier dose after checking pH and CYA.
If it clears then returns green within days: CYA is too high, phosphates are feeding new algae, the filter isn't running enough hours, or free chlorine drops to zero between visits. Address the root cause, not just the symptom.
For severe algae, yes — add a polyquat or copper algaecide 24 hours after shocking (not simultaneously, as high chlorine degrades algaecide). For routine maintenance, shock alone is sufficient when done correctly.