Koi are cold-water fish and handle winter well — provided the pond is prepared correctly. Most winter losses are caused by preventable mistakes: overfeeding into cold water, inadequate aeration under ice, or running circulation systems that disturb the thermal stratification that keeps fish alive.
The Temperature Threshold
At 10°C (50°F), koi metabolism slows significantly. At 4.5°C (40°F), digestion effectively stops. Feed wheat germ only from 10°C down to 4.5°C — one to two times per week, small amounts. Below 4.5°C, stop feeding entirely. A koi that consumes food it cannot digest stores undigested material in its gut through winter, which ferments and causes internal damage. Koi swimming slowly near the pond floor in winter are not hungry — they are in a natural semi-torpor state.
Aeration Is Non-Negotiable
Dissolved oxygen drops as water temperature falls — but bacterial decomposition of waste on the pond floor still consumes oxygen. More critically, if ice forms over the entire surface, gas exchange stops: oxygen cannot enter and toxic gases (CO₂, methane, hydrogen sulphide from anaerobic bacteria) cannot escape. Run an air stone or pond bubbler near the surface to maintain an opening. Never completely shut off aeration in winter.
Do Not Break Ice by Force
Striking ice with a hammer or heavy object sends a shockwave through the water that can kill or severely stress fish. Instead, place a pan of boiling water on the ice surface to melt a hole, or use a dedicated pond de-icer (floating heater). A hole 15–20 cm in diameter is sufficient for gas exchange in most residential ponds.
Circulation — Slow or Stop
Cold water is denser than warm water and sinks. In winter, a natural thermal gradient forms: coldest water at the surface, slightly warmer (around 4°C) at the bottom. Koi instinctively move to this warmer bottom layer. Running a waterfall or high-flow circulation continuously disturbs this gradient and forces cold water to the bottom where the fish are resting. Reduce pump speed to minimum in late autumn, or switch to a small surface pump that circulates only the top layer. If temperatures will drop below freezing, turn off waterfalls entirely — ice forming on rocks can block flow and crack liner.
Autumn Preparation Checklist
Before temperatures drop below 10°C: net the pond to catch falling leaves (decomposing leaves spike ammonia in spring), perform a 20–30% water change to flush accumulated nitrate, service the filter (clean mechanical stages, check UV bulb), inspect pumps and seals, and remove any submerged plants that will die back and decompose. A clean pond going into winter means a clean pond coming out of it.
Spring Restart
Do not feed until water temperature holds consistently above 9–10°C for several days in a row — not just warm afternoons. Restart filtration 2–3 weeks before feeding resumes so bacterial populations re-establish. Test ammonia and nitrite before the first feeding; winter sludge decomposing in spring can produce a temporary ammonia spike. Feed wheat germ only, once per day, until temperatures are consistently above 13°C.