Koi need to be in water above 74 F for thier immune systems to actually work properly. Medicines and treatments for any problems do not work at 65 F. Digestion completely stops at 45-50 F. If they eat and have food in thier digestive system it will rot and they will die. Koi thrive in water temps above 75 F.
Trout thrive in cold water. The colder the better, down to 40 F. When they are thriving, koi cannot do anything but lay there dormant. When Koi Carp are thriving, Trout are floating belly up, dead. This is not a it depends subject.
