Shock (shocking a pool)
Also called: pool shock, shocking.
Shock, in pool care, is a level of free chlorine and not a product: shocking means raising free chlorine to a high level, about 40% of the pool's cyanuric acid (CYA), and holding it until a specific problem clears. The bag labeled shock at the store is just chlorine, often with extras.
Why it matters
Shock when there is a reason, not a date on the calendar: visible algae, combined chlorine above 0.5, or after a heavy bather load, a storm or reopening the pool. Keep free chlorine in range for your CYA and there is usually nothing to shock. A weekly bag is not neutral. Cal-hypo shock adds calcium, which builds up until it scales your surfaces and heater. Dichlor shock adds CYA, which accumulates until your normal chlorine stops working. Plain liquid chlorine raises free chlorine without adding either.
In numbers
The shock (SLAM) level is 16 ppm at CYA 40 and 24 ppm at CYA 60, reached with plain liquid chlorine and held, not dosed once.