Creating ice-free areas
Continue to keep an area of water free of ice to release noxious gases. Use a pool heater or place a saucepan full of hot water on the ice and allow it to melt through. Do not break thick ice in the pool, because of possible harm to the fish. Rubber balls, or large logs, floating on the water’s surface will absorb the pressure of ice in any concrete pool, such as an ornamental pond, which can otherwise expand so much that it can crack the perimeter.
Pumps in winter
Pumps not removed in the autumn can usually be operated throughout the winter. They must be positioned well below the iceline to avoid mixing the colder, upper layer of water with the lower, relatively warmer, zone.
This may mean raising the pump intake and the return flow to a higher level than normal, but take care that the pump is still below the likely depth of any ice. Check if you have a really hard frost.