In most parts of the United States, fish can be left in the pond during the winter, providing the depth of the pond is adequate (18 inches minimum or deeper in harsher climate zones) with little or no water circulation. Fish will gather in this deepest area of the pond where the water temperature remains more stable and go into a state of hibernation during the winter months. Note that some types of fish, such as fancy goldfish, bubble eyes and lion heads, are sensitive to cold weather and should be brought indoors.
As winter approaches, you should monitor pond water temperatures daily. When the water temperature falls below 39°F, you should stop feeding the fish altogether. Some may worry about not feeding their fish, but you can rest assured that your fish will naturally rely on stored energy reserves built up during the summer to sustain them throughout the winter months.
Don't forget your plants!
Most aquatic plants (with the exception of tropical lilies and water hyacinths) can be left in the pond through the winter. Trim back all dead foliage, and then submerge the plants in the bottom or deeper section of the pond to prevent the plants from freezing. Hardy water garden plants will come back year after year. Consider bringing tropical lilies and water hyacinths indoors for the colder months.