While goldfish are capable of surviving in a variety of temperatures, they do best between 65 and 72 degrees Fahrenheit. Once temperatures get below 50 degrees Fahrenheit, goldfish go into hibernation and can potentially die.
Goldfish may survive in ponds that are partially frozen on top. Although risky, owners with outdoor ponds may opt to allow goldfish to go into hibernation during cold winters. During warmer months, goldfish should be fed a high-protein diet to build up fat stores. When the temperature drops to around 60 degrees, goldfish should be given easy-to-digest food until they stop eating due to hibernation. If the pond freezes on top, goldfish huddle in the warmer depths, but a hole of no less than a foot should be kept clear of ice to allow oxygen into the water.