In the winter, koi and goldfish enter a semi-dormant state called torpor. It’s not quite hibernation, and they are still living and breathing.
They do not need food when water temperatures are consistently below 55 degrees, but they are still exhaling gases that we need to get out of the pond. Disturbing the water surface with a waterfall or a pond aerator as the temperatures drop below freezing is critical for fish health so those gases don’t get stuck under ice. (See our tips for putting holes in your iced-over pond, as well as our advice for preparing fish for their winter dietary needs.)
Should I take my fish inside for the winter?
Generally, no. Most fish can weather the cold just fine thanks to their amazing biological safeguards.