Winter Survival Guide for Pond Fish

How to keep your fish healthy, oxygenated, and stress-free all winter long

Winter presents a unique challenge for backyard and natural pond owners, especially when it comes to protecting fish. As temperatures drop, your pond transforms into a calmer, slower ecosystem where fish conserve energy and oxygen levels can decrease. 

With the right preparation and equipment, your pond can remain healthy and stable throughout the winter, providing your fish with a safe environment in which to rest until spring returns. This winter survival guide walks you through what happens beneath the ice, how feeding habits change, and the key tools that help your fish thrive in the winter.

What Happens to the Pond in Winter?

As winter settles in, your pond ecosystem begins to change. The densest water, around  4°C, sinks to the bottom, forming a warmer layer where fish gather for protection. A thin layer of ice forming on the water surface is normal and actually helps insulate the water below. The primary concern is when the pond is completely covered with ice, which can trap gases and limit oxygen exchange.

During this time, plants and beneficial bacteria slow down significantly. With less photosynthesis and slower breakdown of organic material, oxygen levels can drop to critical levels if the pond isn’t properly maintained. 

How Fish Adapt to Cold Water

Fish don’t truly hibernate, but they enter a low-energy rest state to survive winter. Their metabolism slows dramatically, their movement decreases, and they rest quietly near the bottom to conserve energy.

You’ll notice:

• Very slow swimming
• Long periods of resting
• Staying deeper in the pond (little surface activity)
• Less interaction and feeding interest.

This behaviour is normal; it is how fish naturally survive cold seasons in ponds and lakes.

How Feeding Changes in Winter

As pond water cools, fish metabolism slows dramatically. Their bodies become less efficient at digesting food; thus, feeding amounts and frequencies must be adjusted to minimize food waste and prevent water pollution.

During the active months, we highly recommend our nutrient-filled Pond Pro Premium Koi Food (available in 3mm and 5mm pellets) to support strong growth, colour, and activity in Koi and goldfish. For trout, we recommend our nutritious Pond Pro Trout Food (also in 3mm and 5mm pellets). 

However, when fish are outdoors year-round, their feeding habits change as temperatures drop. Below 10°C, their ability to digest food declines significantly, and by around 4°C, digestion virtually stops. Any food offered at this point may sit uneaten at the bottom, affecting water quality, or worse, fish may attempt to eat it and suffer internal issues that can become fatal.

Here’s a simple winter feeding guideline:

• Above 10°C: Continue feeding as usual
• Between 10°C and 4°C: Reduce feeding gradually and choose easily digestible formulas
• Below 4°C: Stop feeding entirely

When in doubt, use your fish’s behaviour as a guide. If they are sluggish and not coming to the surface, they are conserving energy; it’s best to let them rest until the spring.

Pellet Size and Type to Use

Fish size affects pellet choice, with a general role being that the pellet size should be roughly half the size of the fish’s mouth.

• For small fish under 6” in size, feed  3mm Floating Koi or Floating Trout Food
• For larger and mature fish that are 6” or bigger, we recommend our 5mm Floating Koi or Floating Trout Food

Rich in proteins from fish meal and wheat germ, Pond Pro’s Premium Koi Food provides a balanced diet that supports growth and development. It is enhanced with essential vitamins and minerals, ensuring your koi receive the nutrients they need to thrive. The 3mm size pellets are perfect for koi of all sizes. The 5mm Premium Koi Food is suitable for mature koi that are 6 to 8 inches in size or larger.

Our Floating Trout Food is designed to ensure optimal feeding and less waste. Our formula meets and exceeds the nutritional requirements of trout, promoting vibrant health, rapid growth, and excellent water quality. Select from 3mm or 5mm floating pellets to suit the size of your trout. The 3mm formula is best suited for smaller trout, ranging from fingerlings to those up to 6 inches in length. Larger trout require a larger 5mm pellet size to ensure they receive sufficient nutrients.

Why Oxygen Matters in the Water

Cold water naturally holds more oxygen, but winter brings conditions that can reduce availability:

• Plants slow down their oxygen production
• Beneficial bacteria slow down
• Organic matter still breaks down slowly

The solution: maintain gas exchange and circulation with a pond aerator and/or de-icer. Together, they prevent oxygen depletion and keep your fish healthy all season.

Aeration introduces additional oxygen to your water, while a de-icer maintains an opening for gas exchange. Together, these keep your pond healthy and oxygenated year-round. Aeration circulates oxygen actively and can be used to create an opening, while a de-icer prevents toxic gases from building up under winter ice.

Aeration

Consistent aeration is one of the most essential tools for preventing winter fish kills, especially when ice forms and gas exchange becomes limited. A reliable aeration system ensures that oxygen is continuously moving through the water column. It maintains a safe opening in the ice, allowing harmful gases to escape and giving your fish the best chance to rest comfortably and stay healthy until spring.

Can-Air systems offer a safe and efficient method for maintaining oxygen circulation in cold weather, eliminating the need to submerge electrical equipment directly in the water. They keep the pond surface free from floating hardware, meaning nothing interferes with natural ice formation, and your setup stays clean and unobtrusive.

Each system delivers clean air through bottom-anchored diffusers that sit securely on the pond floor. Their stainless-steel bases self-right, so you can lower them into place, making them perfect for fall setup before freezing conditions arrive.

Key winter-ready advantages:

• No electricity in the water for safety
• No floating components
• Self-righting stainless steel diffusers, easy install
• Cabinet and insulated line upgrades prevent winter damage
• Available in 120V and 240V to support your setup

Whether you have a small koi pond or a large natural pond, Can-Air has a system designed to support fish through winter.

Can-Air Koi Pond Aeration System

• Ideal for backyard koi and goldfish ponds
• Effective up to 5-7 ft deep (9 ft with HiBlow Upgrade Kits)
• Helps maintain oxygen and gas exchange under ice
• Comes with 1, 2, 3, or 4 diffusers, with an outdoor cabinet upgrade

Can-Air Eco Pond Aeration System

• Great for small to medium ponds up to 30+ ft deep
• Comes with 1, 2, or 3 diffusers, with an outdoor cabinet upgrade

Can-Air SS Diffused Aeration System

• For medium to large ponds (minimum 5 ft depth)
• Available from 1 to 10 diffusers for custom layouts
• Available with an outdoor cabinet upgrade

Can-Air Mega Diffused Aeration System

• For expansive pond and lake applications
• 1 to 6 diffusers per system
• Available with an outdoor cabinet upgrade

By pairing natural winter fish behaviour with the right aeration support, you set your pond up for a healthy, stress-free winter season. With continuous, gentle oxygen circulation and safe ice openings, your fish can settle into their resting phase and conserve energy until the spring returns.

De-Icing

Even with good aeration, severe cold can cause the surface to freeze over with ice. When that happens, toxic gases build up and oxygen exchange drops, stressing fish and aquatic life. A de-icer helps maintain an opening in the ice, allowing your pond or lake to “breathe” all winter.

A well-placed de-icer:

• Prevents full surface freeze-over
• Allows harmful gases to escape
• Helps oxygen enter the water naturally
• Reduces winterkill risk in fish ponds
• Supports healthy overwintering conditions for wildlife

De-icers don’t warm the entire water body, they simply circulate slightly warmer bottom water upward to keep a hole open. This protects the deeper, stable cold-water spot that fish rely on. Available in ½, ¾, and 1 HP, and 120V, 240V or 220V, with the ability to open a space in the ice up to 90’ in diameter (1 HP unit at -6.6°C to 1°C), we have a de-icing solution for you.

We also offer a 300 Watt Pond De-Icer for keeping a small hole in the ice during winter months. This is intended for a small backyard pond or koi fish pond. It comes with a built-in thermostat so that it only turns on when needed, accompanied by an LED light that signals when the de-icer is in use.

Winter Pond Prep Tips

A little preparation before freezing temperatures helps your pond stay stable:

• Scoop out leaves and debris
• Trim dead or dying aquatic plants
• Use beneficial bacteria products leading up to winter, and again in the spring

We offer a range of beneficial bacteria products designed to improve water quality, enhance clarity, and reduce muck buildup, including our NaturalClear Pond & NaturalClear Lake Clarifiers, and our specialized line of Muck Pellets.

Bacteria Product Comparison

With thoughtful winter care, your pond becomes a safe and tranquil sanctuary where fish can rest and emerge strong when warmer weather arrives. Understanding seasonal changes, adjusting feeding routines, and supporting oxygen levels with aeration and de-icing equipment are the keys to preventing stress and winter losses. By preparing early and monitoring conditions throughout the season, you’re not only protecting your fish, you’re setting your pond up for a vibrant, healthy spring. As always, our team is here to help you choose the right products and equipment for your pond, so you can enjoy peace of mind all winter long.