You've probably enjoyed the peace and tranquility of your koi or goldfish pond all summer and fall. But now winter is here and the temperature is falling fast. What do you do to protect your beloved aquatic pets? Not to mention your invested time and money?
The two main issues a pond owner needs to consider during the winter months are temperature and oxygen. In general, koi and goldfish deal extremely well with cold-water temperatures. Obviously, temperature does become an issue if your pond freezes completely. A surface layer of ice on your pond by itself will not cause problems for your fish. As a result of this surface layer, however, oxygen exchange between the atmosphere and the water is curtailed. This lack of oxygen exchange could result in dangerously low oxygen levels in the water as the fish continue to respire. For this reason our main focus will be on preventing the occurrence of freezing and the resulting low oxygen levels.
What Can You Do?
There are several options available to pond owners for managing their ponds during the cold winter months. The option best suited to any situation is dependent on many factors. Some of these factors include the climate of the area, the size of the pond, the pond biomass (biomass is a standardized measure of the amount of living things by weight. A single 1 pound fish would have the same biomass as four 1/4 pound fish), and of course the cost of dealing with the situation.
The first option would be to essentially do nothing. Depending on the climate of your area and the size of your pond this may be a viable choice. In warmer areas where the temperature rarely dips below freezing for periods longer than a few hours, and little more than a thin sheet of ice forms around the edge of your pond, additional measures are not usually necessary. In colder areas this method could work for you given the right situation, such as if you have a deep pond with a small amount of biomass. The reason for this is that when the water temperature drops, fish, being poikilotherms (cold blooded), have a reduced metabolic rate. This decreased metabolic rate means they use less oxygen. In addition, cold water holds more oxygen then warm water. These two factors combined allow the fish to survive under a total layer of ice. This is why wild fish survive freezing of ponds and lakes.
The second option would be to prevent the formation of the ice on the surface of the water. This can be achieved in two ways; first by heating the water and secondly by agitating the water. There are several ways of heating the water to prevent the formation of surface ice. Large-scale in-line heaters are available for virtually any sized home pond and are capable of maintaining pond temperatures above freezing even in the most severe conditions. They are available in different styles and use different sources of energy, from electricity to propane.
The downside to these heaters is that they are expensive not only to buy but also to run. And as you'd probably expect, heating water in an outdoor pond is not very efficient. Solar heaters can be adapted for this purpose. While these are less costly to run they are still expensive to purchase and are not readily available. In addition, a prolonged period of overcast skies could hinder their operation. Finally, there are floating and sinking pond heaters. These are electric heaters that do an adequate job of providing localized heat in small ponds. If your overall goal is to heat your entire pond and your pond is large, these are not for you. If, however, you are simply looking to keep an area free of ice they will work very well. It is important when using this type of heater in a lined pond that a guard be used to prevent damage of your liner.
It is much more difficult to freeze moving water than standing water. For this reason keeping your pond water moving can prevent the formation of ice. Depending on the climate and the amount of flow, a filtration system may provide enough movement to hinder ice formation.
The important consideration in agitating the water is to break the surface tension of the water, which will inhibit ice from formation. One method of doing this is through the use of a fountain or other device that sprays water over the surface of the pond. A second method would be to use air bubbles to break the surface of the water. Either of these techniques is quite effective.
The downside to these methods is that under extremely severe conditions they may not be effective. Additionally, using too much aeration in very cold water can cause oxygen supersaturation that could result in problems for your fish if conditions should change.
Your final option would be to remove the animals from the pond. Many people have large tanks or aquaria set up in their basements or garages where they keep their animals during the winter months. This is a good way to deal with the cold weather and may be the only way to manage the problem in extremely cold climates where severe weather is the norm. The obvious drawback to this approach is the large space requirement and the expense of the tanks and filtration systems.
Once the animals are out of the pond there are a few ways to deal with the pond itself. First, the pond can be drained. This may be useful in very small ponds but in most cases it is not feasible. This also requires completely restarting the pond including the biofilter when warmer weather returns. Second, the pond may be treated like a swimming pool and covered; a floating icebreaker is a nice addition to help prevent ice damage from occurring. Third, some sort of ice prevention (aeration, spray, or heat, as mentioned above) could be employed to prevent ice damage where possible. And finally, leaving the pond as is until the spring. The major thing to remember here is that dirt and debris will still accumulate in the pond and that in severe weather there is the potential for ice damage to occur. If you do leave your pond full it is best to continue running your pumps and filters, which will still need to be maintained.
All of the above mentioned techniques have their advantages and downfalls. For most people living in a temperate climate, a combination of surface heating, spraying and/or aeration will provide adequate protection during the winter months.
There are other considerations one should make when preparing for the winter months. As water temperatures drop so does the fish's metabolism. This means the fish require less energy to carry out the same functions. For this reason, the fish require less food. If you continue to feed the same amount you did during the heat of the summer you will end up with one of two things: fat fish or a tremendous amount of wasted food at the bottom of your pond. Either result is undesirable.
In fact, many fish will go off feed all together in cold temperatures, which is why they are able to survive under a complete sheet of ice without being fed. Bacteria will break down any accumulation of excess feed or wastes in the pond. This bacterial breakdown uses oxygen so it will no longer be available to the fish. Therefore, it is in your best interest to have as little wasted food in your pond as possible. To this end it is also advisable to be sure to clean your pond and your filters prior to the arrival of cold weather. This will help reduce the consumption of oxygen by the bacteria leaving more available for the fish.
One final note. Many people keep aquatic plants in their ponds. Many of these plants are tropical or sub-tropical in origin and may require special care during the winter months. Pond owners should consult their aquatic garden specialist to find out what appropriate care to give to their valuable plants.