Overwintering a Garden Pond: Winter Care for Koi and Goldfish

How to prepare a koi or goldfish pond for winter: when to stop feeding, keeping a hole in the ice safely, pond depth, debris removal and the spring restart.

Pond fish such as koi and goldfish are ectotherms (cold-blooded), so their body temperature and activity follow the water. As a pond cools through autumn and winter, the fish slow dramatically and need very different care from summer. The two pillars of winter care are managing feeding as the water cools and keeping gas exchange going under ice, all without disturbing fish that are resting near the bottom.

Stop feeding as the water cools

Cold fish cannot digest food properly, so feeding must wind down as temperatures fall. For every 10 degrees Celsius drop in temperature, a fish's metabolic rate roughly halves, so the fish need less and less energy and eat less. Below about 10 degrees Celsius (50 degrees Fahrenheit), switch to a wheatgerm-based food, which is easier to digest, and offer only a small amount until the fish naturally stop feeding altogether.

Keep a hole in the ice for gas exchange

When a pond ices over completely, oxygen cannot enter and carbon dioxide and other gases cannot escape, which can harm fish overwintering below. Keeping an opening in the ice allows this gas exchange. The safe way to maintain a hole is with a pond de-icer or floating heater, often combined with an air stone positioned near the surface.

Manage pumps and depth

In winter, fish rest in the deepest, most thermally stable part of the pond. Running a pump that pulls water from the bottom can circulate the warmer deep water up to the freezing surface and supercool the zone where the fish are sheltering. Many keepers raise the pump higher in the water column or reduce circulation so the bottom stays undisturbed. Adequate depth is the foundation of winter survival: koi ponds are commonly built 3 to 5 feet deep to prevent freezing solid, giving fish unfrozen water to shelter in beneath the ice.

Clear debris before winter

Remove fallen leaves, debris and dying plant material before the pond freezes. Left to rot under ice, this organic matter decays, consuming oxygen and releasing gases at the very time gas exchange is restricted, which worsens conditions for dormant fish. Netting the pond in autumn keeps leaves out in the first place.

The spring restart

When the water warms again, koi come out of dormancy and feeding resumes gradually, starting with easily digested food. Spring is a vulnerable time: fish are at their weakest after winter and disease risk rises as the water warms, so reintroduce normal feeding and full filtration slowly and watch the fish closely.