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Must-Know Koi Facts For Winter Care & Survival

By November 29, 2016March 22nd, 2022Ponds
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What’s colorful, elegant and can command hours of your gazing time as they glide and dart through water?


These jewels of the pond have an interesting history and are bred in such a variety of colors and patterns that you could name and pick them out individually just like any family member. In fact, koi can easily become part of the family since they can live as long as 30 years in your pond when well cared for. In a lake or naturally stream-fed pond, koi can live past 100, and some have been recorded as living to over 200!

Koi are fascinating for many reasons, and they provide pond owners with years of joy and entertainment. That’s why it’s so important to understand their habitat and how to care for them, not only when they’re offering gazing pleasure, but especially during cold winter months when they’re most vulnerable to things like disease and freezing temperatures.

These are some must-know koi facts so you can protect your fish family all winter.

Koi Don’t Actually Hibernate

Hibernation is a period of low or no activity that some animals go through during the winter. First, the animal must build up fat stores to sustain it during this period without food. During hibernation, the animal’s metabolism, heart rate and respiration slow down to help it conserve energy.

Koi, on the other hand, typically retreat to the bottom of the pond when temperatures drop, to where the water is slightly warmer. Their metabolism does slow, which is why you should not feed them during the winter, since excess food and waste can build up as decaying organic matter that can throw off the balance of your ecosystem and even breed disease.

But this period of reduced activity is not true hibernation, and if you keep a heater running in your pond, you may not see much of a downshift in activity at all.

Temperature Swings Are Stressful

Whether you keep a heater running or let your koi wait out the winter at the bottom of the pond, it’s important to know that temperature fluctuations of 20 degrees or more can actually make fish more prone to infection.

Your first line of defense when it comes to protecting fish from temperature swings is to invest in a simple pond thermometer. No pond owner should be without one. Not only will you know exactly when to take other care measures, such as adding cold water bacteria, or switching (or eliminating) feeding, but you can keep an eye on the temperature to be sure that it’s status quo.

It’s not uncommon for temperatures to change dramatically during late fall and early spring. It may be a refreshing 70 degrees during the day but still dip below freezing at night. This can be especially challenging if you have a small pond. Larger ponds tend to be less susceptible to temperature shifts. If pond temperatures concern you, it may be time to consider a heater during the night, or running your waterfall during the day to keep water circulating and temperatures throughout more stable.

Fish Need Oxygen

They may not breathe the way we do, but they still need oxygen to survive. The good news is that cold water holds more oxygen than warm, and since fish slow down, they will require less oxygen during winter anyway.

But that doesn’t mean you’re off the hook! Problems can still arise, especially if the surface of the pond freezes over. The finite amount of oxygen beneath the surface will eventually be used up, and if none is circulating in, that is a recipe for disaster.

During the winter, keep an aerator running to ensure oxygenation, and if your pond surface is in danger of freezing over, use a deicer to keep an open space.

Koi Cannot Survive Being Frozen

Water itself is fascinating, and a source of many optical illusions. One of those illusions is making it appear as though your fish are frozen solid in a block of ice. That can happen if the pond surface is frozen and you can see through to the fish below. In their inactive state they may appear as if frozen in the ice itself.

But that’s not the case! So if you think your fish managed to survive one winter of being frozen, and don’t take proper precautions the next, thinking they’ll be just fine, you may be in for quite an unpleasant surprise.

Koi Are More Susceptible To Infection During Winter

Fish may slow down during cold months, but many parasites do not! In fact, some are even more active in cold water. Some species of ich, trichodina and costia thrive quite enthusiastically in icy water.

Couple that with the fact that the koi’s immune system is in an essentially non-functional state during winter and you’ll understand why fish are more prone to disease at this time of year.

To protect your fish, take the appropriate measures to balance your pond. An adequate amount of salt in the water can help rid it of harmful parasites. But be careful – too much salt can be just as dangerous. The key to protecting any pond is balance.

You can also reduce the chance of disease and infection by ensuring that you’ve cleaned out decaying organic matter, including dead plants and fallen leaves, before they become a breeding ground for bacteria.

But not all bacteria are bad! There are actually beneficial bacteria that you can use to counteract infection rather than cause it. Beneficial bacteria will “eat” organic matter that can otherwise decay and become a problem, reduce toxic ammonia and nitrites, and eliminate nitrates, phosphates and excess nutrients, all of which can improve your koi’s chance of maintaining good health.

Koi Are Unique Among Fish

If you own koi, you’re not crazy for thinking they recognize you when you stop by the pond. Koi are actually highly intellectual, can recognize the person who feeds them and can even be trained to eat out of your hand.

Koi are omnivores. They’ll eat anything from plants and other fish (if they’re small enough) to bugs, watermelon, eggs and peas. If you receive koi as a gift, it’s considered to bring good luck. And if you do have koi in your pond, you know you can never have just one. They’re too addictive! But perhaps more importantly, they do prefer the company of other koi – not so different from the rest of the family after all.

So if you enjoy and adore your koi, then be sure to take care of them and create a healthy environment for them to survive the winter months. A few simple steps can protect them now and for years to come.

If you need help with your pond or are looking for someone to add a professional touch, let us know. We’re equipped with everything from the right pond supplies and experience to keep fish healthy, to a full fish hospital to help when things go wrong.