First off, why should you want to keep a hole in your ice covered pond? Most people will agree that keeping your pond open in the winter will let built up gasses from rotting plants and leaves escape. This is so that the water that your fish are in does not accumulate toxic levels of ammonia and other decomposition gases. There are a number of ways to provide for these gases to escape.
1. The use of an air pump is widely used. Just a small air pump from the pet department, along with an air stone will do the trick. By placing the air stone near the surface of the water it will agitate the water sufficiently enough to keep a hole in the ice. It's a good idea to house the air pump outside in a somewhat insulated box so that the pump uses dry air or at least air that will not have too much moisture in it to avoid condensation in the tubing. If condensation occurs the moisture may freeze on cold days effectively plugging the bubbler. It may be advisable to suspend the bubbler off the bottom of your pond to avoid stirring up any muck or debris that may have settled out over the year. The water that moves up with the air will keep a hole open as well as helping to strip out any gases.
2. Another way is to use a small, with respect to G.P.H., water pump. Having a pump sitting a few inches below the water's surface will also keep an area of the pond from freezing (moving water doesn't freeze). As mentioned above, keeping the pump off the bottom is important. First off you don't want to disturb any of the muck that is on the bottom. Secondly, your pond water is getting some heat from the ground. The higher the pump is, the less you are helping this heat to escape.
3. Some people keep a hole in their pond by running their waterfall all winter long which would eliminate the need to purchase another pump. Again, as with No.2, your pump should be raised up off of the bottom of the pond. And in severe weather you'll have to keep an eye on the ice build up on the falls itself. If the ice builds up too much it might divert the pond water outside the pond and drastically lower the pond water level.
4.The use of some type of electric heater such as a heater for a stock watering tank is another way to keep a hole in the ice. A note here: any of these heaters are NOT MEANT to keep your ponds water up to summer time temperatures; most in fact, were not meant to be used with ponds at all. These heaters are designed to keep water troughs open for livestock during the winter. Some of them have guards on them so that they won't come in contact with a pond liner. The one drawback to this type of heater is the amount of electricity that it uses, usually around 1000 to 2500 watts. This can be helped along with a thermostat that will shut off the unit when, for example, the air temperature rises to above 40 F. (4° C.) but may prove to be prohibitively expensive to run. Also, you will need to clean the heater frequently with vinegar or Lime-away to reduce Calcium-Magnesium deposit build-up.

Another type of heater makes use of the heat from one or two regular house type light bulbs to keep a hole in the ice. This setup consists of a light fixture mounted to some type of container, which floats on top of the pond. The advantage to this type of heater' is that it requires far less electricity, as little as 75 watts. This type of heater can be bought on the Internet, in pond stores or made at home. One idea for a light bulb de-icier' can be found by entering the following in your favorite search engine: Greg's floating deicer. Another popular one is Steve's Heater' a drawing of which is appended to this FAQ. A final note: Electricity and water don't go together very well. With a heater, pump or any electrical appliance always have them plugged into a G.F.C.I. (Ground Fault Circuit Interrupter) which you can purchase for an extension cord if you do not have one already installed for the circuit. Be sure that your light bulb system uses a G.F.C.I. It is also advisable to use a three wire system in addition to the G.F.C.I.
David, Ronaye and Steve {{gwi:2110322}}Image by: SteveC50
Light bulb deicer