For colder climates it is likely necessary to heat your pond to keep the water temp above 34* which is where fish begin to die.
I came across this pool Solar Heater and thought it may even work for a pond. You can use more than one in series. The heaters need to be placed after your filter so debris does not build up inside the tubing.
You may even be able to create a diy version since it is little more than black tubing in an enclosed clear dome. You might be able to also use it as a heat exchanger with water circulating slowly through it in a closed loop. If you have ever left your garden hose on the lawn on a sunny day you know how warm the water inside can get. You could also create a heating circuit by adding a timer to a pump that only works during the day, or use a low powered solar fountain pump and circulate water through the heater when the sun is shining (at no additional costs).
The only thing that would make it not work is if your water flow is too fast to aborb the heat as it flows through the solar collector. Also, those nasty icy or snowy days are not likely to be very sunny. So it would be more of a method to use to reduce need of an electric heater, not necessarily as a replacement.
