we had a huge ice storm up here in new york and lost power for 4 days. got damn cold. i bought a generator after the second day. only ones that died were some neon tetras i threw in as feeders. fish started acting wierd, i was quite happy when they started acting normal again.
I'm sorry for your losses. It could have been the water change. I did one an did not lose power but starting losing my expensive plecos. I had some old declorinator or not enough and these fish went quickly. If I wasn't there to fish them out when they died; it would have caused and ammonia surge that would kill the other fish.