I assume you use a dechlorinator with each water change. The only thing that can kill fish that fast is chlorine. Chlorine is toxic in very small quantities, on the order of 0.05 ppm chlorine. City water can also have what is called Super (Shock) Chlorination. Assuming all other water parameters are OK.
If the city ever did a 'super chlorination' they will not tell anyone. The chlorine will be gone in a day or so. You have to have a test kit for this.
Just something to think about as a lot of folks have lost their entire tank to this. That's why it is recommended when adding your de chlorination that 2 times the amount be added just in case this ever happens. Folks don't realize this as it happens fast and the chlorine is gone.
This is true only if you live in a third world country or believe false stories read on the internet!
Perhaps you did not bother to actually READ and understand the report you linked?
"Shock chlorination is a process used in many swimming pools, water wells, springs, and other water sources to reduce the bacterial and algal residue in the water. Shock chlorination is performed by mixing a large amount of sodium hypochlorite, which can be in the form of a powder or a liquid such as chlorine bleach, into the water. The common advice is that the amount added must raise the level of chlorine to 10X the level (in parts per million) of chloramines present in the pool water-this is "shocking". A lesser ratio is superchlorinating. Water that is being shock chlorinated should not be swum in or drunk until the sodium hypochlorite count in the water goes down to three ppm or less. Commercial sodium hypochlorite should not be mixed with commercial calcium hypochlorite, as there is a risk of explosion. Although a verb for superchlorination, "shock" is often misunderstood (through marketing and sales language) to be a unique type of product."
Even a suspect source like Wiki knows the difference!
Treating pools to be safe to swim is NOT the same as treating drinking water and this article specifies that the water is not safe to drink!
If you know anything about chlorine, it is not a fast killer as it does burn the soft tissues like gills and that does slowly choke them to death.
As for the "super chlorination" you need to read the water regs and do a bit of thinking! Super chlorination as you describe will also kill people, so do you get lots of people dying in your neighborhood when they do the "super chlorination" or are you just passing on bogus information?
I assume you are new and have never done much reading about water treatment, so I would suggest you first read a Consumer Confidence Report (CCR) as required to be available to consumers. It will list what treatment your water company uses, the minimum and max amounts tested in the system over the last year as well as the maximum amount allowed!
Please stop repeating bad information or post up better info than what you have!
Super chlorination is used in swimming pools!