University of Illinois Extension

Koi and Goldfish Pond Winter Care

It’s easy to avoid winter kill in your garden fish pond by simply keeping a small area of the pond ice-free for the exchange of gases with the atmosphere, said Jeff Rugg, a University of Illinois Extension horticulture educator.

“Water is a unique substance. At about 40 degrees F. it is denser than water warmer or colder, so the 40 degree water settles to the bottom of the pond. It forms a puddle that if left undisturbed will not mix with the colder water above it,” said Rugg. “The ice floating on the top of the pond insulates the water from even colder air above it. Koi and goldfish can safely spend the winter hibernating in the 40 degree water.

“Winter temperatures can remain very cold for many months in northern climates. There’s only so much water in a pond. There can only be so much oxygen in the water. If ice caps over the pond for longer than a few days, the fish, other animals and any decaying organic material may eventually consume all the oxygen and the fish will suffocate. “

This result is referred to as “winter kill” and occurs commonly in natural ponds and our over-stocked backyard Koi and goldfish ponds are much more susceptible.

“Air bubblers and small water pumps can be used to keep small pond areas ice free in regions that the ice only forms for a few days at a time,” he said. “Do not allow them to mix the lower 40 degree puddle of water with the colder top layers.

“Any flow of water across the pond that disturbs the bottom puddle of 40 degree water will eventually lead to a fish kill. They cost less to run than pond deicers, but do not work when the air temperature drops below the teens for extended periods of time.”

In colder regions, pond deicers are not meant to warm the pond, but just to keep a small area free from ice cover for the exchange of gases with the atmosphere.

“Most are set to turn on around 34 degrees and off at around 40 degrees,” he explained. “Keep the heating coil on the deicer clean. Mineral deposits can build up on it, insulate it from the water, and cause the unit to fail. The heating coil can be dipped into lime-a-way or vinegar to dissolve the deposits.”

Low wattage deicers of less than 200 watts, work in the same regions that air bubblers work. Deicers with wattages over 1,000 watts work better in colder climates where ponds freeze over for weeks at a time. Low wattage deicers are not more energy efficient than high wattage deicers.

“All heating devices work the same, because physics is involved,” said Rugg. “It takes a specific number of calories of heat energy to raise the temperature of an object a specific number of degrees. A single candle over a period of time will generate the same number of calories as a blow torch over less time.

“If the object being heated is sitting in the freezer and therefore giving up heat at the same time it is gaining heat, it will take longer to warm up and if the freezer is cold enough, it may not warm up at all. So, smaller wattage deicers run longer in northern ponds to do the same work as a bigger wattage deicer does in shorter time. But, the same amount of electricity is used and the electric bill will be identical too. In the coldest weather, a larger wattage deicer will work to keep the ice open that a smaller deicer cannot.”

To make any deicer more efficient, use something to insulate it. If the deicer can be set in the corner of the pond, so it can be covered with a plastic dome or plywood shelter, so there is some sort of a roof over it that traps lost heat, it will work more efficiently and to colder temperatures. Snowfall will not bother the deicer, since it is just above freezing, the snow will melt around it.

“If the fish are gasping for air at the surface, this may indicate either low oxygen or excess toxic gases, like hydrogen sulfide or carbon dioxide,” he said. “Find a way to enlarge the hole for better gas exchange using another bubbler or heater. Do not pound on the ice. If necessary, use a garden hose to melt a larger hole. Use dechlorinator if the hose water has chlorine in it.”

If the 40 degree puddle is left in the pond, the ice cannot freeze too thick until the puddles gives off some heat. If the water has been mixed, so there is no puddle, the heat is gone and the ice can replace the water very quickly.

“Remember, only liquid water can give its oxygen to the fish,” Rugg said.