Oh. I think I may know what happened. If I am reading this correctly, you have always used tap water before for your freshwater tank, Then the last water change you used RO/DI water. Is that correct?

If so, then it was a pH shift that probably killed the fish. The tap water around here is around pH of 8 and the RO/DI water is around pH of 7. if you did a 50% watre change that would have been enough to burn the fishes gills and that would also explain their behaviour prior to death. If you have a pH test kit, test the pH of your tap water, your tank water and your RO/DI water. I think you may find your answers there.

Let us know what you find.