Yes — pond fish need a hole in the ice, but not for the reason most people assume. Fish don't suffocate because ice blocks air from getting in; they die because ice and snow seal the pond so decomposition gases can't get out and no new oxygen is produced underneath. Keeping even a modest opening in the ice all winter is the single most effective defence against winterkill.
What actually happens under the ice
A Canadian pond in January is a sealed system. Fish, bacteria, and decomposing leaves and muck all consume oxygen through the winter, and once snow covers the ice, sunlight can't reach the plants and algae that would otherwise produce a trickle of new oxygen. Oxygen levels fall steadily from freeze-up onward, while carbon dioxide and other decomposition gases accumulate with nowhere to go. In a shallow pond with a mucky bottom, the oxygen can run out before spring — and the owner discovers dead fish at ice-out. That's winterkill, and it's most common in exactly the ponds people assume are safe: small, shallow, well-fed backyard koi ponds.
Why a hole in the ice fixes it
An opening in the ice is a gas-exchange vent: decomposition gases escape and atmospheric oxygen dissolves in at the open surface. It doesn't need to be large — it needs to be permanent. A hole that refreezes in a February cold snap is a hole that wasn't there when the fish needed it most. That's why the reliable methods run all winter rather than relying on a one-time opening.
The right equipment for the job
A Kasco de-icer keeps a defined opening ice-free all season by pushing warmer bottom water to the surface — the standard choice for backyard fish ponds, and it pairs with a C-10 thermostat panel so it only runs when temperatures require. For deeper ponds and stocked dugouts, diffused aeration run through winter does the same job from the bottom up, with the compressor onshore. One caution for koi ponds: position equipment so it doesn't circulate the very bottom of the pond, where fish overwinter in the warmest water — our team can advise placement for your setup.
What doesn't work
Never smash a hole in the ice — the shock waves stress and can injure dormant fish. Pouring hot water makes a hole that refreezes within hours. Floating objects to "keep a spot open" freeze in place. If the pond is already sealed and you must open it mid-winter, set a pot of hot water on the ice and let it melt through gently — then get permanent equipment in before it refreezes.
Fall setup checklist for fish owners
Before freeze-up: remove leaf litter and excess muck (decomposition is the oxygen thief — beneficial bacteria through the fall helps), stop feeding once water stays below about 10°C as fish metabolism shuts down, confirm your fish have at least two feet of depth below expected ice, and install the de-icer while the water is still open — our installation timing guide covers when, region by region.
Frequently Asked Questions
How big does the hole in the ice need to be?
Modest but permanent — even a metre-wide opening maintained all winter provides gas exchange for a typical backyard pond. Consistency matters far more than size.
Can koi and goldfish survive winter under the ice?
Yes — they go dormant near the bottom and overwinter well, provided the pond is deep enough (at least 2 ft below the ice) and a gas-exchange opening is maintained.
Why did my fish die even though the pond had a hole for most of the winter?
"Most of the winter" is usually the answer — a hole that refroze during a cold snap, or heavy snow cover blocking light, can push oxygen below survival levels in days. Run equipment on a thermostat so cold snaps are exactly when it works hardest.
Is winterkill a risk in large ponds and dugouts too?
Yes, especially shallow, nutrient-rich ones. Stocked dugouts on the prairies commonly run diffused aeration all winter for this reason.
Questions about your pond's winter setup? Email Info@fountaindepot.com or call 1-855-752-5795 — advice backed by 50+ years of installation and service experience.
September 08, 2026
August 18, 2026
