You also need to use some form of floating device to keep the surface of the pond from freezing. In extreme cases you may want to consider a heater, be sure to get one designed for a plastic aquarium so as not to melt the liner. This will allow them to go into their hibernation and not result in any fish loss. Remember that fish do not have digestive systems and need the bacteria that only grows in warmer water to help digest food and expel waste. Good luck and remember how beautiful the ponds looks when it's covered by either snow or ice this winter.
Spring is only a short time away.