Can we stop worrying about why they are flushing the lines and help the OP with their tank?
It is my understanding that the process of removing chlorine from a tank by a dechlorinator utilizes oxygen. So if there's a ton of chlorine/chloramine to remove then it can rid the tank of oxygen and cause the fish to suffocate. Shrimp having a smaller oxygen demand were probably not affected.
When I worked at an aquarium the county would send us a notice when they were adding extra chlorine and we would simply not do water changes for the 2 weeks while it occurred, and then continue once it was finished.
I don't think there is anything you need to do to your aquarium, but I would wait until they are finished before doing anymore water changes.
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I note two things about this which do seem silly. One is that the OP was six months ago and they are not likely to even care about the discussion at this point!
The second is the idea that there is a "ton" of chlorine in the water.
The total point of having science folks in charge of treatment is to make certain that the levels are carefully controlled. This is done routinely and very carefully and tests are taken at many different points where the water is used. It is not just a job for some dope to dump a bunch in and let it flow! Samples are taken, they are send to state approved labs to be tested for all the things that we need to make water safe to drink and chlorine content is just one of those tests.
If people are really interested in getting a true answer to what work is done on their water, they can do some simple things to find out. One is to look for the Consumer Confidence Report that is required of almost all providers who serve more than a few people.
This is a small snip of what the local Austin report shows for chloramine content during the years test reports. It shows the low, high and maximum amount allowed by state and federal regs.
If you want to see the total report for 2019, it is here:
If you want to know what happened in your water during the past year, do a look for the CCR for your supplier and read it!
When people look for the truth, it is easy to find but it is much more common to just spout a bunch a garbage!!