The effects of adding calcium to a Swimming

The effects of adding calcium to a Swimming

 Using calcium chloride flakes - when I add it to a swimming pool  my chlorine level goes to 0 And I find that it’s very hard to get my chlorine level back up, usually I have to shock the pool or even put more than a shock treatment, not sure why this is happening. I take care of 10 commercial condominium pools and  I added calcium two 4 Pools and all 4 Pools within two hours the chlorine levels dropped to zero??