I live in northern WI, and here the pond needs to be at least 4 or 5 feet deep to avoid freezing, and you need to keep the ice open in a small area, and even then it's not a sure thing. I know someone who has a 2000 gallon pond who overwinters his fish outside, and he always loses a few, especially around February-April, when the weather gets weird. I put my gf and koi in flitered, 100 gallon cattle troughs. You can get them from Fleet Farm or most hardware or feed stores. If you keep them somewhere where it gets cool, around 50 degrees, they will just go into "hibernation." Attached garages or unheated basements work perfectly for this. Then you literally leave them alone till spring (when the weather gets and stays over 55 degrees or so). The most important element to overwintering this way is to NEVER FEED THEM. If they get food in their systems, then go back into hibernation, the food doesn't get digested, and the fish die painfully. I would definitely reccommend not overwintering them in your pond if your weather is anything similar to northern WI (for reference, I live in a horticultural Zone 3, meaning it gets -30 or -40 in the winter), because of all the sad stories I've heard. I do love my method, though. It's been 5 or 6 years now, and I have never lost a fish. Good luck!