More and more pond owners are leaving their filter system running all year round now, through out the winter. There are many ways of doing this without freezing your poor fish. One way is to raise the pump to nearer the pond surface and restrict the flow from the pump to half of what you would have it pump out in the summer. In the summer you would, as a rule, have the pump as far away from where the filtered water is coming back into the pond as possible. If you plan on keeping your pump and filter system running throughout the winter, it's advised that you place the pump as near as you can to where the filtered water re-entering the pond. The reason for this is simple. When the pond water is very cold or icy, your fish need a 'survival' layer that is found at the bottom of the pond. The survival layer is always around 4 degrees C. If the pond pump was at the bottom of the pond instead of at the top and pumping water from the survival layer, the fish could freeze to death as colder water from nearer the surface would replace the survival layer of 4C.

I personally have always shut down my pump and filter system for the winter. I was interested how many don't and if you've had any problems leaving the system running during the winter months?

Thanks for the info Spudgun. How deep should a pond be so that fish survive the winter?

Hard to say to be honest. It depends where in the world you live and how cold the winter is. If you have koi in your pond then I'd say, no shallower than 24 inch deep. The deeper the better though.