Koi can live in a wide range of temperatures, from 34 to 90 degrees Fahrenheit. However, the extreme temperature ranges aren't necessarily the best for their health and happiness.
The most harmful temperature range for Koi is from 40 to 50 degrees Fahrenheit. This is known by Koi enthusiasts as "Aeromonas Alley". At this temperature range, Koi may be more susceptible to bacteria and parasites in the pond water.
Through heating your pond, it is possible to pass through the "Aeromonas Alley" temperature range much more rapidly. In fact, "Aeromonas Alley" may be skipped altogether, if you keep the pond temperature above 50 degrees Fahrenheit during fall, winter and spring. For large or deep ponds, try the GC Tek Max-E Term Gas Koi Pond Heater.