Think of it this way. Shock is a verb, not a noun. It is something you DO to a pool, not a special chemical you add. It means bringing the chlorine level up to a high enough level to either destroy combined chlorine or kill algae. Also, it is NOT something that needs to be done weekly. It is something that needs to be done on an 'as needed' basis (when combined chlorine is over .5 ppm, after heavy rains, after heavier than normal pool use such as a party, etc.)

All shocking once a week does is help increase the pool store's "bottom line" (as does buying expen$ive lithium $hock!)