Pond Deicers are used in your garden pond during the winter months to allow toxic gases to escape and oxygen to be replenished. Depending on your "fish load" in the pond, the ice and debris accumulation, you must provide oxygenation for your fish, otherwise the fish will become stressed and "winter kill".
Selecting a De-Icer is much easier now that there are several styles on the market, each created to fit different applications.
The larger ponds require the more powerful units, drawing 1250 watts. The problem with that much draw is that your electric bill will increase by about $75.00 or so a month depending on where you live in the northern hemisphere. For small to medium ponds with a normal to light "fish load", there are floating units that do not use as much energy and you can use more than one unit if your pond exceeds 1500 gallons.
There are even natural pond de-icers now that resemble rock formations, should you want a natural look. The problem with the natural look de-icers is that you may need several to keep up with the ice and these units do not melt whatever ice is already on your pond.
You can even add a pump to keep the water circulating and provide a larger ice free surface. Just be careful not to place it near the bottom or near the de-icer to prevent debris from being pulled into the pump.
Inspect your de-icer regularly to make sure that a circuit breaker has not been tripped or for any non working reason.