Well, ironically, and sadly, my fish in my heated, filtered, 5 gallon tank just died (during the -degrees this week). I checked his water as well and it was very cold, even with the heater. My fish in my 1 gallons are still alive and doing well, and their water is warm, due to me using a room heater more effectively.
My water heater, is just not good enough I guess. It has kept the water warm, but at other times his water just gets very cold, well it is automatic. Maybe it was other issues that killed him, other than the cold, he had his gills wide open when I found him if that makes a difference. His fins are full, no rot, or anything else I can see as another reason that would have killed him!