Paul, I don't use a heater anymore. Well, a deicer that is just to keep a hole open in the ice. The fish just go dormant for several months. I can see why you wouldn't want to risk your koi though. It is a bit of a crap shoot as to what will happen. I have lost many fish over the last several years during winter including some spendy koi. Now I just get my koi from Walmart. I love them all the same.
The last three winters we have put a pump in the deeper end then run the hose to the shallower end and 'lofted' it by about 2' so that it arcs to the pond introducing oxygen and keeping the hole open most of the winter though not all. If it is really cold the open water closes up. I have two burned out deicers in there right now. I need to get rid of them (didn't realize they were defective until too late) and get at least two new ones. If the ice stays closed up for too long all the fish die. No way for the gasses to escape, including from their exhalations. They disappear into a cave under some really large rocks in the side of the pond. Rocks are over 3' tall and 2' wide. Natural rock so odd shaped.
