If you added 5 pounds of chlorine, and saw no chlorine bump, then yes, I think there is something (probably a nascent algae bloom) that is creating a demand on the chlorine. I'm not saying there was anything in the city water to begin with, but I am suggesting that it only takes a day or so with a chlorine level of zero for algae to start growing. That's why I want you to test your chlorine levels as I described above, to rule out or confirm a chlorine demand.
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