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What Happens to Pond Fish in Minnesota Winters? Expert Tips for Cold-Weather Care

A Real-Life Winter Pond Survival Story

When Tom and Lisa moved into their dream home in Prior Lake, they were thrilled to find a beautiful koi pond in the backyard. The previous owners had assured them the pond was low-maintenance, and for most of the year, it was. They spent summer mornings drinking coffee beside the shimmering water, watching their koi glide gracefully beneath the surface.

Then winter arrived.

As Minnesota’s first hard freeze set in, Tom noticed the water had iced over completely. Concerned about his fish, he grabbed a shovel and broke through the ice. The next day, the pond was frozen solid again. Still worried, he broke through the ice again, unaware that the shockwaves from pounding on the ice were stressing his fish even more. By the end of the second day, he noticed several fish had died.

Afraid to lose any more fish, Lisa reached out to us at Superior Ponds. She wanted to know what they were doing wrong—and more importantly, how they could prevent it from happening again the next day. After inspecting their setup over the phone through some pictures, it was clear that while they had a stunning water feature, they lacked the critical winterization steps needed to ensure their fish’s survival.

If you’ve ever been like Tom and Lisa and wondered what happens to your pond fish during Minnesota winters—and how to help them thrive through the cold—this guide will explain the biological changes your fish undergo, the common dangers they face, and the best DIY maintenance practices to keep your pond ecosystem healthy.

Prefer to listen instead? Listen to Andrew and Emma of the Ponderings Podcast discuss this article below!

koi and goldfish in frozen Minnesota pond through winter

How Do Pond Fish Survive in Frozen Water?

Minnesota winters are brutal, with temperatures regularly dipping below zero. Unlike warm-blooded animals, koi and goldfish are poikilothermic, meaning their body temperature matches their surroundings. As water temperatures drop below 50°F (10°C), these fish enter a dormant state, significantly slowing their metabolism and activity levels. They consume less oxygen and rely on stored body fat to sustain them during this period of dormancy.

This dormant state, called torpor, allows them to conserve energy until spring. According to PetMD, during such times, koi will spend more time at the bottom of the pond with minimal movement.

However, for fish to survive in a frozen pond, several key conditions must be met:

  1. Oxygen Exchange Must Be Maintained – Fish still need oxygen, even in dormancy. A fully sealed pond surface traps harmful gases and depletes oxygen.
  2. Water Quality Needs to Be Stable – Decomposing organic material (like leaves and waste) releases toxic gases under the ice, which can be deadly.
  3. Proper Depth Is Crucial – Shallow ponds freeze solid, while deeper water allows fish to remain in a stable thermal layer.

The good news? With the right winter preparation, your pond fish can emerge healthy come spring.

The 5 Biggest Winter Threats to Your Pond Fish

1. Oxygen Depletion Under Ice

One of the most overlooked dangers in winter is oxygen depletion. A completely frozen pond without proper aeration can suffocate fish trapped below.

What to Avoid:

  • Letting the pond freeze over without maintaining an open hole.
  • Relying on waterfalls alone to provide oxygen, as they can freeze solid and create ice-dams.

What to Do Instead:

  • Install a Floating De-Icer – These devices keep a small hole open in the ice, allowing gases to escape.
  • Use an Aerator – Placing an aerator in a mid-section of your pond prevents oxygen depletion. According to the University of Minnesota Extension, properly aerated ponds have significantly lower fish mortality rates. We’ve personally seen this in ponds all around the Twin Cities and across the state.

2. Poor Water Quality from Decaying Debris

Even in winter, water quality remains crucial. Organic debris like leaves and fish waste decompose, releasing ammonia and hydrogen sulfide, which can be lethal under ice.

What to Avoid:

  • Skipping Fall netting, allowing excessive amounts of leaves to sink.
  • Not performing a quick Fall cleanup, allowing sludge buildup.
  • Assuming cold temperatures prevent water quality issues.

What to Do Instead:

  • Perform a quick Fall Cleanout – Remove excess debris before freezing temperatures set in.
  • Use Cold-Water Beneficial Bacteria – According to Aquascape Inc., applying cold-water bacteria treatments helps break down waste even in low temperatures, reducing toxic gas buildup.

3. Ice Shock from Sudden Breaks in the Surface

Many pond owners, like Tom, panic when they see their pond frozen solid and try to manually break the ice. Unfortunately, smashing through the surface sends intense shockwaves through the water, which can fatally stress fish.

What to Avoid:

  • Hitting the ice with a shovel, hammer, or drill (if you do need to open a hole in the ice, you can carefully pour hot water to create a small hole in the ice)

What to Do Instead:

  • Let the De-Icer Work – If installed early, it will work to keep a section open.
  • Use an Aerator or Pond Heater – Both prevent full ice coverage safely.
  • Monitor Ice Thickness – According to The Minnesota DNR, ice thickness fluctuates with temperature changes, so keeping an open hole requires consistent monitoring.

4. Fish Freezing in Too-Shallow Water

If your pond is too shallow, your fish may not survive the winter. Water freezes from the top down, and if your pond is less than 30 inches deep, there’s a risk that the water column will be too narrow for creating a healthy geothermal layer for your fish to survive.

What to Avoid:

  • Keeping large fish in a pond under 2.5 feet deep.
  • Assuming small fish will survive if deeper sections aren’t available.

What to Do Instead:

  • Ensure a Minimum Depth of 2.5 Feet – Deeper ponds maintain a more stable thermal layer.
  • Consider Bringing Fish Indoors – If your pond is too shallow, setting up a winter holding tank indoors may be the best option (This is another entire sub-topic, please reach out with any questions for temporary indoor fish care over the winter)

5. Choosing the Wrong Pond Care Provider

If you hire a professional to winterize your pond, choosing the wrong one can be just as risky as not winterizing at all. Some providers lack the experience or knowledge to properly prepare your ecosystem, leading to costly mistakes.

What to Avoid:

What to Do Instead:

  • Take advice from an experienced pond maintenance company – At Superior Ponds, we specialize in Minnesota pond care and understand the unique challenges of our harsh winters. A pond contractor from another state may not be familiar with our volatile seasonal weather and temperature fluctuations.
  • If you do hire this out, look for a clear and transparent maintenance plan – structured winterization services, including fall cleanouts, aeration installation, and ongoing monitoring are signs that the company knows what they are doing.

Why This Matters for You

Your pond should be a place of relaxation, not stress. Winter is one of the most critical times for pond health, as small missteps can mean the difference between thriving fish and springtime heartbreak. As Tom and Lisa learned, it takes several intentional steps. Thankfully, we were able to step in and save the rest of their fish and get them through till spring in a properly winterized pond.

By ensuring proper aeration, maintaining water quality, and making sure your pond is deep enough, you’ll protect your fish through the coldest months. And if you’re considering professional help, be sure to choose a specialist who truly understands winter pond care.

Have you struggled with winter pond care before? What’s your biggest challenge in keeping your fish healthy during the colder months? Drop your thoughts in the comments below—we’d love to hear from you!

How Superior Ponds Can Help

At Superior Ponds, we’ve spent years perfecting our winter pond maintenance system to provide stress-free, hands-off solutions for Minnesota pond owners. Whether you need a fall cleanout, aeration system setup, or full winterization services, our team ensures your fish and pond remain healthy all year long.

With our VIP Maintenance Membership, we even offer a Clear Water Guarantee through the season and include winter peace-of-mind check-ins, so you can rest easy knowing your pond is in expert hands. Contact us today to schedule a winterization consultation and protect your pond for seasons to come!