Extension Educator, Horticulture
"Bundle up." It's cold outside. Unless you are into knitting tiny goldfish sweaters, your backyard water garden fish may need a little help to survive the winter. Fortunately according to Jeff Rugg, University of Illinois Extension horticulture educator, it's easy to avoid "winter kill" in your garden fish pond by simply keeping a small area of the pond ice-free.
First a lesson about water and its uniqueness: Water about 40-degrees F, 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. 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 in the 40-degree water.
As we know, winter temperatures can remain very cold for many months. There's only so much water in a pond and only so much oxygen in the water. If ice completely covers the pond for longer than a few days so no new gas exchange occurs, the fish may eventually consume all the oxygen and therefore suffocate. There will be more than ice floating at the top. This result is known as "winter kill" and does occur in natural ponds. However our over-stocked backyard Koi and goldfish ponds are much more susceptible.
Rugg shares his tips to avoid "winter kill".
In warmer regions where ice forms for only a few days at a time air bubblers and small water pumps can be used to keep small pond areas ice-free. However, 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. Bubblers and pumps cost less to run than pond deicers but do not work when the air temperature drops below the teens for extended periods.
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. Keep the heating coil on the deicer clean. Mineral deposits can build up, 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.
To make any deicer more efficient, use something to insulate it with a plastic dome or plywood shelter. The cover traps lost heat and the deicer will work more efficiently and to colder temperatures. Snowfall will not bother the deicer.
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. 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 de-chlorinator 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 "puddle" gives off some heat. If the water has been mixed so there is no "puddle" the heat reserve is gone and the ice can replace the water very quickly to produce flash frozen fish.