I have a 160 gal pond in Zone 4. One winter the bubbler (small aquarium type) worked fine; kept a hole open and the fish survived.. The next winter the line froze up or something and it stopped working and the hole froze over and the fish died.

A friend has a more expensive bubbler that keeps going all winter and is happy with its performance.

I am debating now whether to buy a better bubbler for this winter or move my three fish to a friend's deeper pond for the winter.

In zone 5 a 120gallon pond froze almost to the bottom. I wound up retrieving fishies on Xmas day in sub zero weather. I kept them in a large new garbage can in the garage with a small pump and a bubbler. There were a lot of them and they all survived.

I'm zone 6 and I run my submersiable pump with the water bubbling just under the surface in a 160 gallon above ground stocktank all winter outside. When it's predicted to stay below freezing for an extended period of time I put in a 100 watt deicer. If the ice gets too thick I replace it with a tank heater. In the February 2010, the tank froze from both the sides and top and buldged the bottom. I had fish lose, but from my trying to remove some of the ice.

I use air in some 'still' lily tanks and they do maintain a hole unless we get a prolonged cold spell. I sometimes have to open up 'an air dome' of ice first thing in the morning. Last winter we never had over 3" OR 4" of ice at any one time. When the ice got thicker I would rotate my stocktank heater between the tanks each day to maintain that opening.