It all depends on where you live and how deep your pond is. If you pond is 2' deep (in ground) then you won't need a heater. You would, however need some way to keep a hole in the ice for gas (oxygen, etc.) exchange. A floating heater like those used in a stock tank work great and is what I use. You can also use a bubbler under the water surface to keep a hole open. Koi and goldfish are coldwater fish and survive winters just fine. My ponds in central Ohio have frozen over solid and I never lost a fish. Hope this helps.

A heater is not necessary. It heats the whole pond and uses energy like you would not believe. If you wanted a hole in the pond for gas exchange, a de-icer is much more practical but you only need gas exchange if you have fish.

Assuming the pond is not hard sided like cement and is in ground that would not require a hole in the ice. However, if it is hard sided it is possible that ice expansion could cause a crack. Swimming pool owners get around that by anchoring an inflatable pillow at the water surface that can absorb the pressure of the expanding ice.

Thank you for your responses. I truly appreciate the help.