A few degree change in temperature won't kill fish.
If the water company did work on the pipes a few days before you did the water changes, there might have been extra chlorine/ chloramine in the water that has affected some fish. When the water company does work on pipes they add extra chlorine/ chloramine to make sure nothing is alive in the water. They don't tell you this and the extra chlorine/ chloramine can take a few days before that heavily chlorinated water is used by people. However, if it was that I would expect problems to occur in all tanks that got water changed.
The only time I have had issues deworming fish was if the tanks got overdosed and small fish suffered first. They became nervous and skittish but none died. When I did a water change they started acting normally again.
Does the tank have any big rocks or driftwood in that could displace a lot of water?
If yes, you might have slightly overdosed and they were fine the first time but the second treatment might have been too much for them, especially if they had been exposed to some chlorine/ chloramine around the same time.
