"Shocking" is a pool trade term with many conflicting definitions, but the common thread in most of them is that "shocking" involves quickly raising the free chlorine to a level 2x - 3x normal.
For most people, it's necessary because they are erratic in their pool treatment, and "shocking" can penetrate emerging algal biofilms before they become a serious problem.
One common misconception, which is artfully promoted by some pool chemical companies, is that "shocking" involves different chemicals than regularly treating with chlorine. It's true that trichlor is not a very practical 'shock', but all other forms of chlorine -- cal hypo, bleach, or dichlor -- are.
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