My parents have near 100 outdoor fish (Jap Koi and such) and they keep a small heater in the ponds in the winter. Instead of spending an arm and a leg for the one made for fish...they went and bought one made to keep a cow's trough from freezing. Much cheaper and works great.
My parent's fish will hibernate as well outside but she keeps the heater in there to keep the pond from freezing. She has a time trying to keep them all alive all the time. Something about ammonia levels in the water I think.