How to maintain your koi pond in winter
Q. We put in a garden pond this last fall. Do we need to do anything special for the fish in it during the winter?
A. If you have Japanese koi, or other goldfish, a few precautions will serve them well. If you have that swordfish from your Miami vacation and wanted to have it on hand to show your brother-in-law what a fisherman you are, you'll have problems.
We forget that fish need oxygen too. They're just able to pull it from the water. The main consideration in winter is keeping enough oxygen in the water for the fish. The equation is simple. More fish equals more oxygen needed.
The rule of thumb is for every inch of fish length, you should have a square foot of surface area. An 8-by-5-foot pond with 40 square feet would support 10, 4-inch fish. Or one 40-incher. You can also estimate 13 gallons per inch of fish.
If your fish/oxygen ratio is good, the issue is keeping the oxygen level appropriate in the water, which happens by a process called turnover. When surface water cools to 40 degrees, it becomes more dense and settles to the bottom, bringing dissolved oxygen from the surface. Also, dense water can hold more oxygen, making it perfect to keep the fish alive, as long as there is an opening in the surface for the gas exchange.
Our potentially long winters require a small floating heater to permit open water and fresh oxygen reaching the bottom, or you'll have to physically make and maintain a hole for gas exchange.
Pond heaters are not expensive to run. They generally engage at around 34 degrees and shut off at 40 degrees. If the winter sets in with sufficient ice, you can put a small plywood or plastic tent over the heater to retain more heat and use it somewhat less. The purpose isn't to keep the pond ice-free, but rather only an opening allowing for air circulation.
Don't be tempted to run your filter, or waterfall, during nice winter days. The circulation will redistribute the cold, but not freezing, water at the bottom of the pond and if the weather turns back frigid, your fish will become koi-sicles.
Ensure you stop feeding the fish when the water temperature hits 50 degrees. They will stop eating and food will build up, decaying on the bottom. Plant hygiene is important. Clean up any dead plant material in the pond and on the banks, and remove any tender water lilies or other tropicals and you should have healthy pond fish in the spring.
— Matt Steichmann
• Provided by Master Gardeners through the Master Gardener Answer Desk, Friendship Park Conservatory, Des Plaines, and University of Illinois Extension, North Cook Branch Office, Arlington Heights. Call (847) 298-3502 on Wednesdays or email northcookmg@gmail.com. Visit web.extension.illinois.edu/mg.