Sorry to hear what happened, I always feel disappointed when fish die. When you put so much time and effort and thought into an aquarium, and to have something go wrong, it sucks.
What do you think killed them? Aren't you using that Hagen CO2 system and not a pressurized setup? I think only a pressurized setup has any risk of killing fish, but maybe with a small tank the Hagen could. Do you think the fish might have died from the temperature being too low? I've left my filter off by accident before in my planted tanks, everyone was fine...so I don't know if it was the filter being off that was the problem.
Anyhow, I was only trying to help so maybe you could prepare for this from happening again. I'm always worried about the power going here, but I've never had it happen.