Koi Pond Winter Care
Even though winters are relatively mild here in Arlington Virginia, winter is the season when your koi fish are most vulnerable. To answer common questions about winter koi pond maintenance and koi fish care we turned to our favorite fish wrangler John Bianchi of Blue Ribbon Koi. John is an expert on koi pond design and in koi fish care. He knows the equipment and he knows the fish.
Doing 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.
Bianchi acknowledges that there is an advantage to heating a pond. Keeping the water warm in winter maintains much of the good 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, when 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.
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 going to be expensive. For that reason, most pond keepers in the Virginia-Maryland area shut down their ponds and let fish go into their natural dormancy state.
How to prepare your koi pond for winter
You have three options:
- Fully heat the pond, continue regular maintenance and feed fish as usual. Or,
- 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. Or,
- Run the filtration system but install a valve that shuts down the waterfall. Otherwise, the pump would draw warm water off the bottom of the pond, run it through frigid air over the waterfall and super cool the pond. That could cause sickness among the fish when they emerge from dormancy in the spring. Another reason to bypass the waterfall is because an ice dam could form on the waterfall. If that were to happen, water might back up, overflow and drain the pond dry.
- If you choose options 2 or 3 you will also have to do the following:
Once day time water temperature stays below fifty degrees, stop feeding the fish. 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.
Give the pond a thorough cleaning. It is important to remove all sticks and leaves because, as organic matter decays, it produces gas that can harm the fish if it builds up too high a concentration in the water—especially if the there is ice on the pond.
Add a winter bacterial treatment. Bianchi likes to use one made by Ecological Laboratories called Microbe Lift. There is a winter formula that is one part hardy bacteria that can withstand colder temperatures. The other part is an enzyme that helps break down organic materials that may have dropped into the pond after cleaning.
Bianchi recommends that you keep the filter running. (We’ll explain why later.) If you choose to do so, it is important to monitor the system carefully during winter to make sure that water continues to flow so that pipes or equipment don’t freeze and break.
Remove the ultra-violet sterilizer and store it indoors. This device is part of the filtration system. It kills free floating algae spores that make the water look green. It is a fragile (and expensive) piece of equipment. If the housing for the ultra-violet bulb were to freeze and crack it could trip the circuit breaker and shut down the filtration. That could cause other plumbing or equipment to be destroyed.
Why run the filtration system during winter?
The pond filter is home base for your good bacteria. When you shut down the filter, there’s no oxygen flowing through it and all the bacteria die. So every season you will have to deal with “new pond syndrome.” The bacteria colony will have to re-establish itself. That can take 6 to 8 weeks. There’s always danger of high ammonia levels accumulating during that start up period before the bacteria are able to handle the waste of the fish.
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 as opposed to a dead filter. So you reduce the risk of losing fish to toxicity by shortening that pond start up period.
Koi Pond Maintenance In Winter
As long as the filtration system is running, you will have to do some maintenance throughout the winter. If the filter clogs up and the water flow slows or stops, everything could freeze and break. That’s a common mistake people make: they think that, because the fish are inactive, they don’t have to do any maintenance. You want to see that water is going through the filter and into the pond. This adds oxygen and helps to prevent freeze up.
In severe temperatures, it is important 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 koi ponds 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 do harm to your pond equipment and fish.