i would contact someone from your city/region and ask if any chemicals had been added over the normal amount/in addition, lately.
cities are terrible for adding chems to their water and not informing the population. it could be as simple as upping the dosage of chlorine or chloramine, or adding something else entirely. if you did any water changes while this was in effect and were only dosing with water conditioner for the amount you normally do, it could be why some fish were dying.
we've had this happen around where i live, it wiped out most of a LFS's stock in my city, and another LFS in another city lost a ton of their stock too from another incident.
i always double or triple dose my tanks when doing water changes simply for that reason. it's better to be safe and use a little more, than to be sorry and lose all of your prized fish.