Winter Koi Care: How Koi Survive the Cold (With Your Help)

Koi don’t fight winter; they power down through it. Your job is to not interrupt.

What torpor looks like

Below ~50°F, koi metabolism collapses: they stop eating, movement slows to occasional drifting, and they gather in the pond’s deepest, warmest layer. This is normal — a koi hovering motionless near the bottom in January is overwintering, not dying. They run this program for months on stored energy, which is why fall conditioning (wheat-germ feeding as temps fall) matters.

The three winter non-negotiables

1. Depth. A zone at least 3 feet deep stays above freezing at the bottom in most of the US and gives fish a stable refuge. Shallow ponds in cold states need a pond heater or an indoor overwintering plan.
2. A hole in the ice. Not for oxygen so much as for gas escape — decomposition gases trapped under a sealed ice sheet can poison fish by late winter. An air stone or floating de-icer keeps a vent open. Never smash ice; the shockwaves stress torpid fish.
3. No food below 50°F. The most repeated rule in the hobby because it’s the most broken. The full temperature table is in what koi eat.

Late-fall prep checklist

• Net falling leaves before they sink and rot
• Trim and remove dying plants
• Do a partial water change and gently clean settled muck
• Move aeration shallower; install the de-icer before the first hard freeze
• Stop feeding on schedule — a fat koi in November beats a fed one in December

Winter is also buying season — remotely

Counterintuitively, winter is a fine time to shop: Japanese harvest fish arrive at importers in late fall, and many dealers overwinter purchased fish for you until spring release — ask on their profile’s stated terms, and see how koi shipping works if the fish will travel. Just don’t add new fish to a cold pond; let the dealer hold them, or run a heated quarantine indoors.

Come spring, when the water passes 50°F and appetite returns, you’ll know the winter worked: every nose comes up at the first feeding.