In a pool, how come your not supposed to put in SHOCK and an Algaecide contained by the pool on equal year?

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Answers:    The shock is to get the pH right then the Algaecide will work better when it is.
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Had a pool for 10 years didn't know you be not suppose to. Did it all the time never killed anyone except the algae.
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I am guessing that respectively product works better on it's own, not mixed with the other. just follow the directions to be secure.