Koi is a temperate, not tropical, species. Their optimum temperature range is approximately 65 to 75 degrees Fahrenheit, according to the Southern Regional Aquaculture Center. Koi can also survive temperatures from 32 degrees to 95 degrees Fahrenheit, particularly when the temperature change is gradual, such as due to changing seasons, the Center states.
Ice floating on the top of a pond keeps colder winter air from entering the water. Koi can spend the winter hibernating in 40-degree water and survive, according to the University of Illinois Extension. But the Extension advises keeping a small area of the pond ice-free so that oxygen enters the water. This prevents winter kill.
Cold Temperature and Feeding
Koi appetites decrease as winter approaches. The Cooperative Extension Program at Langston University recommends that for each 18-degree Fahrenheit decrease in temperature, you reduce their food by 50 percent. By winter, koi should receive only 8 to 10 percent of the amount of their summer food and can be fed once or twice a week.
Temperatures that are not optimal can suppress koi immune systems. Outbreaks of koi herpesvirus disease, for example, appear when water temperatures are between 72 and 81 degrees Fahrenheit.