Gases such as methane and hydrogen sulphide that result from the decomposition of decaying plant material are poisonous to fish if levels build up. This occurs in winter if the pond freezes over, trapping gases under the surface.

Never smash the ice as the shock waves can kill the fish. Instead install a heater before the winter, so it can be switched on when a frost is expected, to melt a hole in the ice. Alternatively stand a pan of hot water on the surface to melt a hole.

Expanding ice can damage a pond by putting excessive amounts of pressure on the walls of the pond. A float made from polystyrene or an inflatable ball, absorbs some of this pressure. After removing the float it is possible to siphon out some of the water to create an air pocket below the ice.

Running a fountain can prevent the water freezing in an area large enough to allow gases to escape. If a pump is left in over winter it is worth running it on a weekly basis to keep it operating efficiently.