Unfortunately, the old wives tale of putting a tennis ball into your pond to create an air hole is not particularly effective, as the ball can easily become frozen in place. However, there are several options available depending on your requirements and budget. The simplest involve the continuation of circulation, the insertion of a Pool Heater or simply covering a proportion of the pond.
Moving water will freeze at lower temperatures than still water, so any circulation during the winter months will help prevent freezing. It is important to minimise air contact and not to drag cold surface water down to the base of the pond where the fish will want to rest. The pump should be re-located away from any deep water, a shelf is an ideal location and it can be left to run, without any fittings designed to create surface disturbance.
A pool heater can be inserted into the pond possibly utilising the electrical supply for the pond pump. The heater simply floats on the surface of the pond with the heater element hanging down below water level(our Floating Pool Heater can be found under the Electrical Pond Accessories section). Other heaters are designed to be pumped through in a similar way to an Ultra Violet -see Digitally Controlled Water Heaters section.
Covering a proportion of the pond can also be very simple particularly if you already have a sturdy wooden frame for a net, (if not you will need one). The idea is to cover 1/3 - 1/2 of the pond with a frame on to which you can attach polystyrene or bubble wrap. The frame when in place should not touch the surface of the pond, otherwise this could seal the pond up as effectively as the ice. By creating this cover you are able to trap in some of the heat given off by the pond and keep ice at bay. It is important to remember that the above measures will be effective in most of the UK mainland ponds. However, if conditions become very severe then the ice will build up and breathe holes will need to be made. DO NOT smash or crack the ice, as this can send shock waves through the pond that could seriously affect your fish. The best way is using boiling water to melt holes.