I use a mix of city and reverse osmosis water in order to keep my fish at 7.0 neutral ph, but I have used just city water. What are the parameters of your city water in terms of ph, kh and gh?
My guess (without knowing your previous parameters and current ones) is that your ph was dramatically different from your well to your city (they usually are). And as Pitluvs suggested that you needed to carefully acclimate your fish. That would be particularly true if they died suddenly.
Here is a good link on acclimation:
Other than that it is difficult to answer what is wrong with your city water without knowing what area you live in, the parameters (ph, ammonia levels, nitrites, nitrates etc.) and how your fish died.