Originally Posted by koijunior

Actually koi can easily adapted to temperature around 30 degrees celcius with no problem at all. What hardcore hobbyists concern is the oxygen concentrations. The cooler the water, the more oxygen molecules can be retained in the water. Koi thrives well with more oxygen in the water. Thus, people go to the extent of installing chillers for koi pond, which is a great burden on electrical bill. A much cheaper alternative is to install more air pump outlets in the pond, improving the aeration.