I lost power for 5 days on the last one.The temp in the tank only dropped to a minimum of 57,the house temp dropping to about lower to mid 40's at times. I did lose 2 glolights and a Paradise fish that was new to the tank.I unplugged everything and put a thick blanket over the tank to help keep it warm,just remember they do need some air exchange so don't seal it right up tight.