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Wintertime Considerations for Koi


Goldfish and Koi hate wintertime more than we do. Neither species of fish is indigenous to North America, so here, they merely 'survive' winter. They don't flourish in it. My name is Dr. Erik Johnson and I am a fish veterinarian. My practice is located in Georgia, which typically has balmy winters and very little ice on the ponds. However, winter's effects on the fish seem to be the same whether the pond is merely icy, or iced over. This article intends to inform you of some wintertime facts, but more importantly, this article will seek to use those facts to guide you through a safe and healthy springtime.

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Wintertime “Factoids”

There are certain things you should realize about “Winter” so you can properly interpret certain events and conditions come Spring.

Important Factoid #1
During the winter, the fishes’ immune system is in a predominantly non-functional condition.

Important Factoid #2
Moving water through a thin phase aids it in the gain or loss of heat. This is a simple statement with a lot of meaning. When you pour a cup of hot soup back and forth from one cup to another, you can rapidly cool it for your baby. So a waterfall can dissipate or pick up heat from the pond’s water. In certain climates, such as in the Sierra Nevadas and other desert areas, air temperatures can be very warm by day and ice cold at night. This matters because if your waterfall runs around the clock, you could be warming the water by day, and super cooling it by night. Again, this is a geographical phenomenon and may not apply to you. However, a simple water thermometer could tell you. Temperature swings within the pond over near twenty degrees Fahrenheit are very stressful for the fish. Stress makes the fish more vulnerable to infection. Some people run their waterfalls during the day to pick up valuable free heat, and turn the falls off (making sure to have some other form of submerged pond circulation for aeration) at night to spare that free energy and avoid super cooling.

Important Factoid #3
Turning off your waterfall may spare heat loss at night but can also deprive fish of oxygen and circulation, which may be important. It’s important if water temperatures are climbing to always have some circulation in the pond to maintain sufficient aeration or oxygen exchange for the fish.

Important Factoid #4
Fish cannot freeze into a block of ice and survive.
This is a wintertime factoid that should be destroyed once and for all. I had an interesting experience one winter wherein I had several thirty-gallon fish tanks outside my home for tests I was running. The tanks were populated with Goldfish. They had 300Watt heaters in them. Well, in the dead of winter, the heaters failed because the power did. The tanks closest to the edge of the patio froze solid and broke the tanks. The tank closest to the door also froze “solid” and broke the tank. However, in the last tank, the water in the center of the “block” of ice remained unfrozen and the Goldfish in this pocket of ice water remained alive. Because of the temperatures, the fish were motionless. Anyone looking at them would have said they were frozen solid into the block of ice. However, they were not. I am a big person and I was able to lift the block and shift the fish around, which is how I knew I had to chip open the block and rescue the fish.

Many people see their fish in small ponds, frozen under a solid layer of ice. The fish are utterly motionless due to the cold. They perceive that the fish are frozen in the ice and so they say “My fish were frozen solid and lived!” but this is not the case.

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Important Factoid #5
Another common myth in this hobby is that fish are safer from parasites and pathogens like bacterial infections in the dead of winter because these “bugs” slow down or even stop in icy water. However, the converse is true.
Parasites do not necessarily slow down in ice cold water. In fact, certain species of Flukes are actually MORE active in the icy water of winter, and species of both Ich, Trichodina and Costia will work well in icy water. So it’s an important fact that the fish can be more heavily parasitized in winter than any other time of the year.

With those important facts in your mind, you can now intelligently prepare for winter, and then a successful Spring.

Preparation for Wintertime
To have a very successful springtime, you should properly prepare for wintertime. It is VERY important that the leaf fall from October be removed from the pond. You should NOT leave Fall’s leaves on the pond bottom to rot. This is perhaps the most common cause of a wintertime health failure by spring.
Rotting leaves are not instantly unhealthful for the fish. However, after a whole winter of decay, and with warming trends in the water for Spring, illness is common in ponds with a layer of composting leaves on the bottom.

Recalling from the above that waterfalls can act as supercooling factors for the pond. I recommend NOT running the waterfall during the coldest times of winter. This recommendation is intended to spare the fish from coping with VERY cold water. However, turning off the waterfall causes new difficulties in terms of preparing the filters for freezing. You should drain and dry the pump, and drain and dry the filtration which will not have water moving through it, or it may swell and crack.

Also, if you do NOT run the waterfall in wintertime, be aware that in the Springtime, you will need some sort of water circulation to maintain aeration when the water begins to warm up. Some people run their waterfalls in the warmth of daytime, and turn them off at night. Others will resume the use of the waterfall in the very early Spring and monitor temperatures for radical swings overnight.

When ice forms on the pond, you have an assessment to make. Will the pond be frozen for more than a couple days? You have an action item to attend to if it will be frozen for more than 48 hours.

In Georgia, we usually don’t have ice on our ponds for any 48 hour period. So we don’t worry about de-icers or keeping a hole clear in the ice.
In climates where the pond will freeze and the ice will be over the water for more than 48 hours, it’s a good idea to keep a hole clear in the ice. The best way to do this would be floating electronic trough de-icers. There are pond de-icers, too, which have had the Cattle Water Trough Heater label peeled off and replaced with a ‘twice the price’ pond de-icer label.

Cattle trough de-icers are NOT legitimate pond heaters, and will contribute very little to the warmth of the pond because they only activate at 35-40 degrees and cut off at 45 or so – just staying warm enough to keep some water available in the surface of the ice. Keeping a gas-exchange portal in the ice on the surface of the winter pond is desirable, and where pond ice over is a concern, I recommend a cattle trough de-icer.

You can also make any variety of pond hole-keepers with plastic boxes containing lightbulbs, and more. There is a rumor that breaking the ice with a blunt object like a hammer will harm the fish. I have not found this to be true. Sometimes a person gets in a hurry to go to work and doesn’t have time to nurse a hole in the ice with a hot teakettle. When a guy gets in a hurry and has to break a hole in the ice with a hammer or a brick there’s no harm done.

Using an aerator (column of air agitation) can keep a hole open in the ice if it’s thin, however, stalagmites of ice form over these devices in Canada in the winter. Another method should be found.

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Assessing Health in Wintertime
In winter, fish are very inactive and most owners don’t feed the fish much in winter. So, if the fish are not congregating for food, or schooling around the pond, how can you assess their health?

Direct examination is important. Spend the time on a regular basis to look at the fish as they rest in the pond. Look for clamped fins, redness in the skin or fins, swollen eyes or lips, or fraying, rotting fins. Look for a single fish that is not resting with the other fish. Look for white slimy patches on the fish. All of these symptoms could indicate that the combination of winter’s stress, no food, and possibly an opportunistic parasite is working on the fish.

I have seen cases where people put mirrors on the pond bottom. This way, as the fish swim over the mirror, they can examine the bellies of the fish.
What can the pond owner do to alleviate Winter stress? (I know it's a stressful time for them)
One way to alleviate some of the stress of in wintertime is to apply salt at a rate of three pounds per one hundred US gallons. Using aquarium or pond salt is recommended but some folks use non mineralized cattle salt, water softener salt, or rock salt. Do not use iodized salt. But if you do, it will not kill fish. Salt in certain amounts will kill some or all of your plants. This is especially true for plants which cost much money. If you spent anywhere close to fifty dollars for a plant, or if it’s rare, it will die flat out in salted water.

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