Did the heater actually break or just go bad and trip the GFI? If it broke open, the fish may have been killed by the chemicals inside the heater or even a slight jolt of electricity before the GFI tripped. The temp is probably not what killed the fish, but it's possible. I would put larger odds on the lack of oxygen/circulation due to the loss of power. As the temp drop mostly likely occurred over a few hours, I would also increase the temperature slowly and things should be fine.