Not saying this is your issue, because you were already notified by the water company that a chemical had broken thru.
I worked for a water supplier as a chemist for many years, and because I was the only fish guy there, it became part of my job to go to customers homes whose fish died, and they thought the company was at fault.
100% of the time, after I did a battery of tests, it was obvious the fish keeper killed their own fish. Either out of neglect, kindness (over feeding), not changing water, or changing water without using a chlorine neutralizing agent, adding non-quarantined fish, the list went on and on, many times all of the above.
Water companies are at the mercy of the aquifer, or open water supply, be it rivers or lakes, if the population decides it is not important enough to protect their supply, there will be more and more incidents, and it won't be only fish that die.