We have left our pond running all year long every year for 20 years or so. Here in southern Ohio, we do get some heavy freeze weather. In fact, the 250 acre lake often freezes to well over a foot thick. But my 2 foot deep pond has never frozen all the way to the bottom, and we have not lost fish over the winter to date. (All we ever put in there is common goldfish 4 to 6 inches long, and an occasional bluegill). Usually, the flowing water freezes at the surface, but just keeps flowing out underneath the heavy crust of ice in the waterfall. However, the pipes have frozen shut once or twice over the years, and we had to unplug the pump for about 5 days then.