Cold Fish, Warm Hearts: How Koi Tough Out Michigan Winters
Ever stared at your frozen pond and wondered if your koi are okay under there during one of Wayne County’s brutal lake-effect cold snaps? Or maybe you’re planning a new pond and want to make sure your fish can survive Ann Arbor’s icy season?
In this post, you’ll learn exactly what koi need to live through Michigan winters—from pond depth and temperature to feeding schedules and equipment.
By the end, you’ll feel confident knowing your koi can safely overwinter outdoors with the right setup and pond maintenance.
Can Koi Fish Survive Winter in an Outdoor Pond?
Koi fish can survive winter in an outdoor pond if the pond is at least 3 feet deep, stays partially unfrozen, and has proper aeration. During winter, koi enter a dormant state called torpor. To ensure survival, maintain a hole in the ice for gas exchange and prevent full surface freezing.
How to keep your pond from freezing over.
Our Experience with Pond Fish Survival in Winter
Koi, goldfish, and shubunkins are hardy fish and can tolerate cold water temperatures.
A few key points to help them survive the winter are:
- A reliable pond deicer to keep a hole in the ice for proper degassing.
- A properly sized aerator to help keep the oxygen levels up.
- Removing excess organic matter, like tree leaves or aquatic plant foliage.
- Feeding fish a diet that is high in carbohydrates and lower in protein to ensure the fish put on fat that can be used up during the winter months when they aren’t eating.
- Not feeding the fish when water temperatures are under 50°F.
How Koi Behave in Winter
What is Torpor?
When water temperature drops below 50°F, koi and goldfish shift into torpor, a cold-induced resting state. Their metabolism slows down so much that they barely move or eat.
You’ll often find your pond fish resting together in the deepest part of the pond, all facing the same direction, often near rocks or shelves where they feel protected.
This dormancy helps them survive the winter months without burning too much energy. It also means their needs drastically change—feeding stops, oxygen use drops, and water quality becomes more critical than ever.
Torpor is nature’s way of pressing pause until spring.
Can Koi Freeze and Survive?
Koi cannot survive being frozen into the pond ice and thawed out later. They’re cold-water fish, not ice blocks.
Luckily, a frozen pond surface isn’t always a death sentence.
A stable layer of ice can actually help insulate the water below, creating a more consistent pond temperature that keeps koi fish safe through the coldest weeks.
What matters most is maintaining a small hole in the ice to allow harmful gases like carbon dioxide to escape and oxygen to enter. If gas exchange is blocked, even cold-hardy fish won’t make it to spring.
Pond Requirements for Koi Fish Winter Survival
Ideal Pond Depth
Depth is your koi’s greatest defense against freezing temperatures.
A koi pond in our area should be at least 3 feet deep, though 4 feet is much safer when temperatures plummet after the Oakland County Holiday Lighted Parade and winter really sets in.
Deeper ponds create a thermal buffer, allowing warmer water from the earth to settle at the bottom. This keeps the fish comfortable and the water temperature above freezing, even during long cold spells.
If your pond is too shallow, your koi face greater risk, especially if there’s no pond heater or cover to help retain heat.
Maintaining Oxygen and Gas Exchange
Cold water holds more dissolved oxygen, but that doesn’t mean your pond fish are all set.
A layer of ice traps gases produced by organic matter and fish waste, which lowers oxygen levels and puts your koi’s health at risk.
That’s why gas exchange is essential.
By keeping a small hole in the ice open, you allow oxygen to enter and harmful gases to escape.
You can achieve this with a pond aerator, an air pump, or a pond de-icer—or better yet, a combination of them.
Avoiding Dead Zones
As temperatures drop, leftover leaves and plant matter in the pond become dangerous.
When organic debris decays under the ice, it eats up oxygen and releases toxins into the water. This buildup of harmful gases is one of the most common reasons fish die in winter.
That’s why a fall cleanup is just as important as your spring one.
Remove all visible debris before the first freeze, and consider adding beneficial bacteria in late fall to help break down lingering waste.
Equipment to Use (and Avoid) for Koi Ponds During Winter
Aerators
A pond aerator is one of the most cost-effective tools for overwintering success. It introduces oxygen into the water and creates gentle movement that helps prevent the surface from freezing solid.
Position it about 12 inches below the surface—deep enough to avoid icing over but shallow enough to not disturb the warmer water at the bottom.
With the right setup, a pond aerator will keep a hole in the ice all winter long, giving your koi what they need to survive.
Always have a backup air pump on hand in case of equipment failure or power loss.
De-Icers
A pond de-icer is a smart add-on for especially cold winter months.
Unlike a heater, it doesn’t warm the entire pond. Instead, it activates only when water temperature drops near freezing, then shuts off once it opens a hole in the ice.
This saves energy while still allowing proper gas exchange.
A de-icer works best in combination with a pond aerator or air pump.
Place it in a calm area of the pond, away from strong currents.
If used properly, a de-icer can help ensure your koi fish stay healthy even as ice forms on the pond surface.
Pond Heaters
Full pond heaters can be overkill and aren’t usually necessary in a well-designed koi pond.
While they keep water temperatures stable and allow fish to stay active, they use more electricity and can introduce risks if they fail during a cold snap.
In smaller or shallow ponds, heaters may help prevent freezing, but they’re not a substitute for aeration and gas exchange.
If you do use a heater, monitor it closely and pair it with a thermometer to avoid sudden water temperature drops that could harm your fish.
For most koi pond owners, a de-icer and aerator combo works better.
What Not to Do for Koi Fish Ponds During Winter
It’s tempting to break through thick ice with a hammer or shovel to help your koi.
Don’t.
The vibrations can shock or even kill them.
Instead, melt a hole in the ice using a kettle of hot water or a purpose-built de-icer.
If the pond freezes solid, drill a small hole carefully with a hand auger or a cordless drill with a hole saw bit—just be sure not to puncture the pond liner.
Keeping koi fish safe means knowing when to act and when to let the tools stay in the shed.
Feeding Koi in Cold Weather
When to Stop Feeding Koi Fish
Feeding koi fish in winter isn’t just unnecessary—it can actually hurt them.
As water temperature drops below 70°F, it’s time to switch your koi over to wheat germ-based food that’s easier to digest.
Once it reaches 50°F, stop feeding altogether—this often happens in early November, right after the final farmers market weekends wrap up. At this point, your koi’s digestive system slows so much that they can no longer process food properly.
Continuing to feed them can cause leftover food to rot, harm water quality, and stress the fish.
Use a pond thermometer to keep track and avoid guessing.
Why This Matters
Koi fish eat aggressively in warm months, but cold weather changes everything.
Their slowed digestion means uneaten food sits in the pond, decaying and adding ammonia and other toxins to the water.
This lowers water quality and oxygen levels, increasing the risk of winter fish deaths.
By stopping feeding at the right time, you protect their digestive systems and keep your pond environment stable.
Extra Protection Options for Koi Fish During Winter
Hoop Houses or Covers
If you want to add another layer of winter protection, a hoop house or temporary pond cover can work wonders.
These structures trap warm air above the pond and block wind, keeping the pond surface from freezing as quickly. They also protect koi fish from predators like raccoons or herons that may still prowl during the winter months.
Pairing a hoop house with a de-icer can even allow your koi to remain active longer into the season.
Just remember, if they’re not fully dormant, you’ll need to monitor water temperature and oxygen levels even more closely.
Bringing Koi Indoors
If your pond is too shallow or exposed to strong winds, bringing your koi indoors might be the safest route.
A garage or basement tank kept between 40 and 50°F allows them to remain in torpor without the risk of freezing. Just be sure the setup has a good filter, proper aeration, and minimal light.
Avoid heating the water too much, or they’ll stay active and require food and full filtration.
When spring comes, match the temperature of the quarantine tank with the outdoor pond within 5 degrees before moving your koi back.
Sudden water temperature swings can cause serious stress.
Common Causes of Fish Deaths in Winter
Most winter koi deaths are preventable and come down to one thing: neglect.
If a pond aerator fails, or a hole in the ice closes up unnoticed, oxygen levels plummet and harmful gases rise. In smaller ponds, this can be deadly within days.
Other causes include overfeeding before a freeze, poor water quality, or ignoring signs of stress.
Equipment failure during a power outage is especially dangerous, so always have a backup air pump or generator ready.
Winter doesn’t have to be risky for koi pond owners—it just takes vigilance, prep work, and the right tools.
Preparing Your Pond for Spring Awakening
As the ice melts and temperatures slowly rise, it’s tempting to rush back to normal.
But koi need a gentle return to their active state.
If you’ve kept them indoors, reintroduce them when the outdoor water temperature is within 5°F of their tank. Sudden swings cause shock and can trigger disease.
Don’t feed koi fish immediately if cold weather is still possible—wait until the water stays above 50°F consistently.
Remove covers, clean out any debris, and test the water quality before restarting your filter system.
A thoughtful spring start sets your koi up for a healthy year ahead.
How we clean ponds for clients.
Winter Warriors: How Grass Roots Protects Your Pond Fish
At Grass Roots Pond & Garden, we know exactly what your pond needs to get through Michigan’s longest, coldest months. From installing pond aerators and de-icers to making sure your koi fish stay healthy and stress-free all winter long, we’ve got you covered.
Let us help you build and maintain a pond that’s ready for every season, not just summer. Fill out our contact form today or give us a call to keep your koi safe and thriving year-round.
