How To Winterize Your Koi and Pond Fish

How To Winterize Your Koi and Pond Fish
Winter in the mountains of Niigata are cold.

Some Wintertime Koi Management Considerations
Winterizing Koi and Wintertime Koi Care – by Doc Johnson

This is the time of year that we consider our Koi as being “dead asleep” and we do not worry too much about them because water temperatures are so cool that parasites and bacteria are almost as dormant as the fish themselves.

Indeed, this is an important time of year because what you do (or do *not* do now) sets the stage for your springtime season in March, April and May, which traditionally marks the “Disease Season”.
There are several considerations for this time of year, which I will address individually.

How To Winterize Your Koi and Pond Fish

At this time of year, we should examine the 1-water quality, 2-the ponds’ cleanliness, the concept of 3-springtime feeding, 4-disease prevention and finally, 5-minimizing fish stress during pond start-up.

Water Quality at this time of year is usually very good. Cold water carries much more oxygen than warmer water does. Even with the filters off, oxygen tensions remain high, and very satisfactory for fish. Partially because their metabolism is so slow!

Ammonia can still be a problem in some ponds if the owner is feeding every warm day they get. I saw another pond that was made with a liner which was installed and seamed in two parts, and was positioned over some Septic tank field lines. The ammonia-rich ground water would well up through the seam in the liner, giving the owner a nice 2ppm Ammonia reading, even in the dead of winter! Ammonia testing is very satisfactory in the winter, if you would only **warm** the water in your hand to at least room temperature before testing it. You see, the reagents give falsely low readings in cold water.

Nitrites should not be a problem because Nitrosomonas is very sensitive and will be inactive in the wintertime. If you *freeze* these bacteria in a block of ice, they will be killed, but if you merely chill them to near freezing they will remain in a state of suspended animation until conditions return to more suitable temperatures.

pH is never a sure bet unless your pond is concrete lined, in which case it’s a sure bet that the pH will be high……Still, for those reasons that apply in the summer, periodic checking of the pH will avoid a “crash” in the pH, which can kill fish.

One other area of water quality for your consideration is the formation of Ice on your pond, which will trap gases and other toxins underneath to the detriment of your fish. It has been said that Ice can be permitted to form for a few days without hazard, and I substantially agree. But there is a period which is “too long”.

How To Winterize Your Koi and Pond Fish

How To Winterize Your Koi and Pond Fish

Folks who have left their traditional backyard ponds covered with ice for weeks have lost entire collections of fish. It’s hard to believe that there could be that much gas formation in the dead of winter, but the proof is in the experiences of hundreds of people every winter.
They reason that in nature, ponds freeze over. However, they do not realize that natural ponds are usually larger, less crowded, and may have inflow of springwater or stream feeds.

I urge you to keep a place in the ice clear for gas exchange and observation of the fish. Cattle water trough heaters (caged heaters) are cheap (about 30-50$) and can keep a patch of ice clear all winter for a small investment in electricity. Air blowers and stones may fail to keep ice from forming, in the harsh Northeastern climes. I have seen a regular stalagmite of ice form over the air-cap there, and the benefit is then lost.

Do not break the ice with a concussive blow, in the event that you are caught unprepared and you find your pond frozen. The blow to the ice is supposedly transmitted through the water and will shock and possibly deafen your fish, ruining their appreciation of music. I wouldn’t worry too much about deafening the fish, this ice-whack-and-shock-phenomona has not been seen in real life recently.

Take your time, you have days, even up to a week to open a hole in the ice. Use a hot teakettle, set directly on the ice. Some folks use coffee heaters, but I wonder if they heater could melt through and fall in?

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Dr Erik Johnson

Dr Erik Johnson is a Marietta, Georgia Veterinarian with a practice in small animal medicine. He graduated from University of Georgia with his Doctorate in 1991. Dr Johnson is the author of several texts on Koi and Pond Fish Health and Disease as well as numerous articles on dog and cat health topics.