Yes, koi are cold-tolerant fish and can live in cold water, including outdoor ponds that experience winter temperatures well below room temperature. Koi are a hardy, cold-water variety of ornamental carp (closely related to goldfish) and are actually poor candidates for tropical tanks because they don't do well in consistently warm water without adequate space and filtration.
How cold is safe: Koi can tolerate water temperatures down to around 32-39°F (0-4°C) as long as the pond doesn't freeze solid and there's enough depth and oxygen. In regions with harsh winters, koi ponds are typically built at least 3-4 feet deep so a layer of unfrozen water remains below the ice, giving fish a stable refuge. A small hole or aerator is often kept open in the ice to allow toxic gases to escape and oxygen to exchange, since a fully sealed ice cap can be dangerous.
What happens in cold water: As temperatures drop below about 50°F (10°C), koi become sluggish, their metabolism slows dramatically, and they stop eating or need only easily digestible, low-protein food. Feeding heavily in cold water can cause digestive problems because their gut can't process food efficiently at low temperatures. Many koi keepers stop feeding entirely once water drops into the low 40s°F (around 4-5°C) and resume gradually as spring warms the pond.
Ideal range for activity: While koi survive cold water, they're most active, colorful, and have the best appetite in water between roughly 65-75°F (18-24°C). Rapid temperature swings are more stressful to koi than steady cold, so gradual seasonal changes are generally better tolerated than sudden shifts.
Overall, cold water is a normal and even necessary part of a koi's yearly cycle in temperate climates, but pond depth, oxygenation, and adjusted feeding are key to keeping them healthy through winter.