Agree a small heater to keep your pond at 40 seems more reasonable than bringing tropical fish outside.most of the hard core koi guys will build a little plastic greenhouse over their pond in the winter and run a 2-5kw heater and keep the water in the low 40's and let the fish hibernate. thats probably your best bet and the same size heater would bring water temps up to the 70s in the summer and not kill your electric bill.