One of the most important ways to keep fish alive in the winter is to constantly aerate the water. Even when the surface freezes, the mechanical aeration will circulate oxygen within the pond, keep an ‘blowhole’ open in the ice and allow accumulating gases to escape.
Of course, this is presuming the pond has a section that is at least 3 feet deep so the fish can overwinter.
I recommend this unit for smaller ponds, although there is a larger unit available if needed. It costs pennies a day to operate.