Hi Pat, I don't have Koi but I have a pond with gold fish. I am no authority on this but yes I keep my pond heated. All I do is make sure there is a hole in the ice so any gases from decompositioin can escape or the fish will die. I also leave the water fall running to keep the oxygen levels up. I really want the fish to hibernate. (and you don't feed them till spring) I use a cow tank heater, about 14.00 dollars and it keeps the ice melted around the heater but not the whole pond free of ice. Try the pond forum, I go there for lots of great information from those who are really in the know. There are alot of people with Koi ponds and I think they can help more.

Good luck!
Cheers, Jando

I read a newspaper story last week about a guy in New Jersey who suddenly lost about 20 fish. He claimed they were poisened by the neighbor.