Koi and goldfish are ectothermic (cold-blooded). Their body temperature and metabolism follow the water temperature. As your pond cools, they naturally shift into a slowed-down winter state, often called torpor. That’s why they may act “weird,” such as moving less, reacting slowly, and spending more time in the calmest parts of the pond. They often stay lower down where temperatures are most stable.

At the same time, the pond itself changes. Ice cover limits normal gas exchange at the surface, and a layer of snow can block sunlight from reaching the water. With less light, underwater photosynthesis by plants and algae can drop, leading to less oxygen production. Respiration and decomposition still continue under the ice. 

Fish and natural microbes continue to use oxygen as they live and as organic material breaks down. In smaller or shallower ponds, especially if there is a lot of debris or muck, dissolved oxygen can gradually decline over the winter.

Fish Under Ice: What’s Normal Fish Behaviour in a Winter Pond

1) Slow, steady movement (or long periods of stillness)

What you might see: Fish sitting nearly motionless, with occasional slow fin movement or a gentle glide.

Why this happens: In cold water, koi and goldfish slow down into a winter low-energy state (often called torpor). Their metabolism drops, so they simply do not need to move much. 

2) Fish grouped together in winter

What you might see: Several koi or goldfish gathered in the same general area.

Why this happens: Fish often choose the most stable part of the pond in winter. That usually means calmer water, fewer disturbances, and more consistent conditions. Many fish also naturally congregate in winter, especially in deeper areas.

3) Resting in deeper, stable zones

What you might see: Fish “hanging out” lower in the pond and not near the surface.

Why this happens: Under ice, the water below the surface tends to be more stable. Fish under ice often settle into the deepest, calmest zone and conserve energy there.

Quick note: If fish are repeatedly coming to the surface, hovering right under the opening, or look like they’re struggling, that points to an oxygen issue rather than normal winter resting.

Warning Signs: When Winter Pond Fish Behaviour May Mean Trouble

1) Fish “gasping” at the opening

What it looks like: Fish under ice are repeatedly coming to the ice opening, lingering right at the surface, or making “gulping” motions where the water meets thin ice.

What it can mean: The most common concern is low dissolved oxygen (DO). In winter, ice limits normal gas exchange at the surface, and snow can block light that helps plants and algae produce oxygen. Oxygen can slowly drop as fish and natural microbes continue to use it, especially in small, shallow ponds or those with a lot of debris/muck.

2) Unusual under-ice surface-seeking or restless swimming

What it looks like: Fish pacing under the ice, cruising repeatedly, or making frequent trips to the surface or to the opening/aeration area.

Most common causes:

• Dropping oxygen, which can become more likely later in winter
• Higher waste by-products under ice from decomposition, which can add stress when the pond stays sealed for long periods
• Heavy snow cover that reduces light and lowers photosynthesis, which reduces oxygen production
• In severe low-oxygen conditions, ammonium can also build up because nitrogen cycling shifts under ice, but this is usually a secondary issue compared with oxygen.

3) A sudden change from “normal calm” to “distress”

What it looks like: Your fish were calm and resting last week, but now they’re repeatedly visiting the opening, hovering near the surface, or swimming in a restless, “searching” pattern.

What to do if you see these signs (quick, safe steps):

• Do not smash the ice. Maintain or create an opening gently (de-icer, shallow aeration, or carefully melt a small hole).
• If safe, clear some snow from the ice to improve light penetration.
• If you have access to one, check dissolved oxygen, especially in late winter.
• If fish are clearly distressed, treat it like an urgent issue and adjust aeration to keep a small opening consistently open.

If You See Warning Signs: What to Do Next

Step 1: Keep a small opening for gas exchange

Your goal is a vent hole, not melting the whole pond. A small, consistent opening helps gases escape and supports oxygen exchange.

Safer options:

• Pond de-icer (pond heater): Helps maintain a reliable opening, especially during extreme cold.
• Winter aeration: Can keep a vent hole open and support oxygen levels when set up correctly. In most backyard ponds, winter aeration is usually best placed shallow rather than at the deepest point, so you don’t over-mix the pond.
• Controlled melting (small ponds only): If you need a quick opening and don’t have equipment, place a pot of warm water on the ice and let it melt through slowly. Keep it controlled and avoid dumping boiling water directly into the pond.

Step 2: Don’t smash the ice

Avoid pounding, chopping, or “shock breaking” the ice. Loud, sudden impacts can stress fish that are overwintering below the ice.

If a heater or aerator isn’t an option and you absolutely need an opening for gas exchange, skip the brute force and use a gentler method: pour warm (not boiling) water to slowly melt a small hole instead.

Step 3: Check your equipment using a checklist

If you already have a de-icer or aerator running, verify it’s actually doing its job:

• Is the pump running continuously?
• Are air lines kinked, frozen, or blocked?
• Is the diffuser bubbling where you expect?
• Is the de-icer maintaining an opening, or did it shift away from the target area?

Step 4: If conditions allow, clear snow strategically

If you notice fish repeatedly coming to the surface, lingering near the vent hole, or “gasping,” it can be an early warning sign that oxygen levels are dropping and your pond may be trending toward a low-oxygen event. 

While the general steps in this guide can help, winter conditions vary widely from pond to pond: depth, fish load, snow cover, and whether you’re running a de-icer or aeration all affect the risk and the best next move. 

If you’re concerned, reach out to our team with a few basics (pond size, average depth, approximate fish load, and what equipment you’re currently running), and we’ll help you narrow down the safest, most effective approach for your setup.

Step 5: If fish are actively distressed, get advice fast

If you notice fish repeatedly coming to the surface, lingering near the vent hole, or “gasping,” it can be an early warning sign that oxygen levels are dropping and your pond may be trending toward a low-oxygen event. 

While the general steps in this guide can help, winter conditions vary widely from pond to pond: depth, fish load, snow cover, and whether you’re running a de-icer or aeration all affect the risk and the best next move. 

If you’re concerned, reach out to our team with a few basics (pond size, average depth, approximate fish load, and what equipment you’re currently running), and we’ll help you narrow down the safest, most effective approach for your setup.

Final Takeaway: Always Keep a Vent Hole & Watch for Changes In Fish Behaviour

Winter ponds can look “quiet,” but that’s often exactly what healthy fish are supposed to do. Slow movement, long resting periods, and grouping in deeper, calmer water are normal signs that koi and goldfish are conserving energy in cold temperatures. 

The key is watching for a shift from calm to urgent, such as repeated trips to the opening, lingering at the surface, or restless pacing, which can signal declining oxygen under ice. If you keep one small vent hole open, avoid smashing the ice, and stay on top of simple checks like snow management and equipment function, you’ll greatly reduce the risk of winter stress and winterkill. 

And if you’re ever unsure, reach out with your pond size, depth, fish load, and what you’re running. Our team is more than happy to help!