you live in a seasonal climate, then using pond
heaters during the winter is likely necessary. Also known
as de-icers, the purpose of pond heaters is not to heat the pond and
thereby keep your fish warm. Rather, they are meant to melt a hole
in the ice, allowing toxic gases to be released.
Floating heaters simply float on top of the water, and
when the water begins to ice over, the heater automatically turns
on, keeping a hole in the ice. There are also submersible heaters,
but these tend to use more energy.
If you really do not want to invest in a pond heater,
you can make your own. Do not, however, go out and pound on the ice
or drill through it to create a hole. This can be bad for the fish.
Rather, boil up a pot of water, then set the pot on the ice and allow
it to melt a hole (you might want to hold onto the handle to make
sure that you don't lose a favorite pot to the depths of your pond!).
If you do decide against purchasing any pond heaters, then make sure
that you keep an eye on your pond and create a hole when need be.