Koi fish can survive a cold temperature as low as 0 degrees Celsius. Just make sure that the water is not freezing into solids. Then, that may be difficult for the fish to sustain. The fish also need enough oxygen and water to make it through the winter months. But there are certain things you can do to keep them alive during that time of the year. Do not feed them when the temperature is 50 degrees Fahrenheit. You can also move them to a tank or garage space for the time being. Another great option is to build a greenhouse over the pond. You can also implement solar cover for pools. Furthermore, move the aerator close to the surface.