During the winter season, ponds and pond fish can be mostly left alone, but it is important that fish ponds retain an ice-free patch throughout the season.
If the water temperature stays at or above 50 degrees Fahrenheit, pond fish will remain active throughout the winter and can be fed on wheatgerm foods once or twice a day. But should the water temperature go lower than this, feeding won’t be necessary. Either the pond fish won’t be eating, or else they’ll be satisfying themselves on algae and other food sources within the pond itself.
At temperatures lower than 40 degrees, pond fish are effectively hibernating. Assuming the pond is at least 3 feet deep, fish can rest safely at the bottom of a pond without much risk of being frozen or eaten by opportunistic predators such as herons or cats.
In cold climates it is not normally necessary to feed pond fish at all, particularly if the pond freezes over. Soft, oxygenating plants such as Elodea as well as algae will be eaten by the fish should they need to eat.
If the pond stays fairly warm all through the winter, i.e., above 50 degrees or so, there is no risk of ice forming and the pond fish will remain active. In such situations pond fish can be fed either soft plants or wheatgerm pellets. These are easier for the fish to digest than the high-protein pellets used during the rest of the year. This is important because cold nighttime temperatures can allow food to remain in the fish’s gut for longer than it should. The danger here is that undigested food will rot, and in doing so can cause serious health problems.
Filters and Pumps
In places with mild winters, filters and pumps may be left running, but should be switched off if ice forms or the water temperature drops below 40 degrees.
Filters and pumps disturb a process called stratification that is essential for a pond to successfully overwinter. A stratified pond will have ice at the top, cold water in the middle, and a relatively warm layer of water at the bottom. During the winter fish will rest in this bottom layer, and the ice at the top acts like a blanket, keeping the pond insulated from the cold air above it. If filters and pumps are left running during the winter, this stratification can’t happen, and the pond becomes colder and less comfortable. The fish are at more risk of dying from cold than from the lack of filtration or oxygenation.
Although ice is beneficial in acting as an insulator for the pond, it can also cause problems by preventing the exchange of gases between the air and the pond. Fish and other organisms inside the pond will be using up oxygen and producing carbon dioxide, albeit slowly, so gaseous exchange is essential.
If the pond is covered with ice for just a day or two, then the likelihood of trouble is small, and the pond can be left alone. But if the pond freezes for more than a couple of days, some attempt will need to be made to remove a patch of ice so that oxygen can get in and carbon dioxide can get out.
Ice on the pond should never be smashed, as the resulting shockwaves can stress resting pond fish. An old-school solution that works well is the use of a floating ball such as a soccer ball. Once ice forms, the ball will bob slightly in the breeze, usually keeping the patch of water around it sufficiently ice-free for gaseous exchange. Only when it gets very cold will the ice form all around the ball, in which case hot water can be used to melt the ice around the ball once more. This is repeated as required, ensuring that there is always at least one ice-free area somewhere in the pond.
The floating ball trick works well if the ice is thin, but during long, cold winters a pond heater will need to be used instead. Despite the name, pond heaters are not the equivalents of aquarium heaters! Pond heaters float at the surface and keep a small patch of the pond ice-free. They are not expensive to run, costing about as much to use as an incandescent light bulb.
Most pond plants will be dormant at this time of year. The important thing is to minimize the amount of dead and decaying vegetation inside the pond, since the decay uses up oxygen, and gaseous exchange is a problem once ice forms. During the autumn the pond will have been cleaned and unwanted plants removed, so it’s a good idea to place a mesh across the pond to stop leaves and other bits of debris from falling into the pond.