Koi Pond Winter Care
Winters have become increasingly mild here in Northern Virginia, but winter is still the season when your koi fish and your pond equipment are most vulnerable.
Here are the answers to some common questions about winter koi pond maintenance and koi fish care. We hope this information will help you choose the right kind of company to maintain pond equipment and the health of your fish.
Koi Pond Maintenance – What’s Best for the Fish?
There is disagreement as to whether it is better to let the koi follow their natural cycle and go dormant, or keep them active by heating the pond in winter. Some say it is better for the health of the fish to keep them active year-round.

John Bianchi of Blue Ribbon Koi and Marine says there is an advantage to heating a pond over winter. Keeping the water warm in winter maintains much of the beneficial bacteria on the sides of the pond and in the filter.
The “good” bacteria contribute to fish health by processing waste and maintaining water quality. This is an important consideration because, if you shut the pond down for the season, all the bacteria die in the cold and must be regenerated in the spring when you restart the pond. This start-up can take six weeks or more. And, during that time your fish will be vulnerable to disease.
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Keeping pond fish active all year round in the mid-Atlantic climate zone is possible, but not entirely practical because heating a pond with a large exposed surface area is expensive. For that reason, most people in the Virginia-Maryland area shut down their ponds and let fish down-cycle into their natural dormancy state.
Prepare Your Koi Pond for Winter – Two Options
Winter Preparation One – keep the filtration system running.
This option is an easy do-it-yourself affair.
Fully heat the pond, run the filtration system, continue regular maintenance checks, and feed your koi fish as usual.
With this option, it is important to regularly check the pond filter to ensure it continues to run properly throughout the winter. The filter is home base for the “good” bacteria that clean the water of waste and keep fish healthy.
When you keep the filter running over winter, some bacteria will survive in dormancy rather than die off. In spring, the bacterial colony starts up much faster with a dormant filter than a dead filter. So, you reduce the risk of losing fish to toxicity by shortening that pond start-up period.

Winter Preparation Two – shut down the filtration system.
This option requires the services of an experienced koi pond maintenance expert.
Shut down the filtration system, drain water from all pipes and equipment, and run a floating heater or aeration device to maintain a hole in the ice. Keeping a hole in the ice allows the exchange of air necessary for the health of the fish.
If following option two, you will stop feeding the fish once the daytime water temperature remains below fifty degrees. Their bodies can’t process food properly at that temperature. So, you must stop feeding and allow their metabolism to slow down for the winter dormancy period.
One drawback of option two is that when the filter is shut down, there’s no oxygen flowing through it and all those beneficial bacteria die. So every season you will have to deal with what Bianchi (Blue Ribbon Koi & Marine) calls “new pond syndrome.” The bacteria colony will have to re-establish itself.
That can take 6 to 8 weeks. There’s always the danger of high ammonia levels accumulating during that start-up period before the bacteria are able to handle the waste of the fish.
To counter this possible issue your pond maintenance expert may inject a winter bacterial treatment. Bianchi uses a product made by Ecological Laboratories called Microbe Lift. There is a winter formula that contains hardy bacteria that can withstand colder temperatures plus an enzyme that breaks down organic materials and waste.
Winterizing Your Koi Pond
This point bears repeating: In severe temperatures, it is essential to maintain a hole in the ice. That opening lets oxygen in and gases out. If the pond freezes solid, no gases can escape. Anything that’s breaking down inside the pond creates hydrogen sulfide—and that can be toxic and deadly to the fish.

Keep in mind that most garden pond designs are only three to four feet deep. That small volume of water can hold only limited dissolved oxygen for the fish. In a lake during winter, fish congregate in pockets. If the oxygen starts to deplete in one area, they just move to another area. They can’t do that in a shallow koi pond.
As long as you do a daily visual check, especially in periods of freezing temperatures, you should be able to catch and head off any potential problems before they can harm your pond equipment and fish.
If you’d like to get expert help maintaining your koi pond water quality and the health of your koi fish, schedule a phone consultation with one of our landscape management specialists.
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