Luckily, both goldfish and koi are able to overwinter successfully in outdoor ponds, so long as the water does not entirely freeze (they will congregate in the warmer water toward the bottom). With all of this in mind, you need to regularly monitor your water temperature throughout the year, ideally checking it the same time each day, and making sure to have some methods in place to help them overwinter more comfortably.
Goldfish enter a period of hibernation when temperatures drop and food becomes less available due to the changing of seasons. To encourage your goldfish to hibernate, slowly begin reducing their food intake in the weeks leading up to the first freeze. Once ambient temperatures drop below 50 degrees, you can stop feeding the fish while they hibernate.
