Your MIL actually didn't do anything wrong, by pure dumb luck. You hardness level is low enough that Cal-Hypo "shock" won't raise the CH level enough to matter, and it DOES add chlorine to the water.

Saw a joke on a political forum the other day describing someone's mind as being like a spare bedroom: Always made up and rarely, if ever, used. But she's smarter than you. Just ask her. She'll tell you.

Carl.