How to Winterize a Koi Pond Without Losing Your Fish

At Fontana Ponds & Water Features, we’ve seen firsthand how a koi pond transforms through the seasons, and winter is no exception. Knowing how to winterize a koi pond without losing your fish is one of the most meaningful things you can do for the living ecosystem you’ve cultivated. The good news: koi are remarkably well-adapted for cold weather. All you have to do is support your pond’s natural rhythm through it.

Understand What Your Koi Are Doing

When water temperatures drop below 10°C (50°F), koi enter a state of torpor. Their metabolism slows, they stop eating, and they settle at the pond’s deepest point where the water stays most stable. This isn’t distress; it’s biology working exactly as intended.

What koi need during dormancy isn’t warmth. They need adequate depth (minimum 90 cm, ideally 120 cm or more), stable oxygen levels, and clean water free of toxic gas buildup. Understanding this reframes the entire winterization process.

Learn more about creating a balanced ecosystem pond.

Start With Fall Cleanup Before Temperatures Drop

The single biggest threat to overwintering koi isn’t cold; it’s decomposing organic matter trapped under ice, which depletes oxygen and releases ammonia. A thorough fall cleanup in October, before the first hard freeze, makes all the difference.

• Net the pond or skim daily during heavy leaf fall
• Remove dead and dying aquatic plant matter, but leave submerged oxygenating plants
• Clean out filters and check pump operation
• Test water for ammonia and nitrite, and address any imbalances now rather than in February

Aeration is Non-Negotiable

Once temperatures fall consistently, your pond’s gas exchange becomes critical. An aerator or pond bubbler is the most reliable tool for maintaining an opening in the ice and circulating oxygen throughout the water column. A floating de-icer works well alongside an aerator to keep that opening clear during cold snaps.

One important caution: if you run a waterfall feature in freezing temperatures, monitor it closely. Ice formation can redirect flow and potentially drain your pond overnight. Never break ice by striking it. The shockwave travels through the water and can injure or kill dormant fish.

Stop Feeding When the Water Cools

In the shoulder season between 10°C and 15°C, transition to a wheat-germ based food, which digests more easily in cool water. Below 10°C, stop feeding entirely.

This feels counterintuitive to most pond owners, but dormant koi don’t need food. Undigested food in cold water creates toxic ammonia and can be fatal. On an unseasonably warm winter day, resist the urge to feed. One warm afternoon isn’t enough to restore koi metabolism.

Keep Watching Through Winter

Winterization isn’t a one-time task. Check weekly that your aerator or de-icer is maintaining an open hole. Watch for foul odours near the pond surface, which signal anaerobic decomposition below. Leave the fish undisturbed; tapping or agitating them during torpor causes unnecessary stress.

Your koi pond is a long-term investment in tranquility, and it deserves the same care in January as it does in June. If you’re heading into your first winter with fish, or you have questions about your pond’s depth, aeration setup, or ecosystem health, we’d love to help. Reach out to us at 778-990-9773 to talk through what your specific pond needs to carry its living ecosystem safely through the season.