Fish can survive the freezing winter months because their metabolism changes and they go into a dormant stage, but only if feeding stops by November 1 and resumes April 1. Any feeding done later in the season than this (when water temperatures dip below freezing) will stress the fish, rot in their bellies and more than likely kill them because they won’t be able to digest in this state of dormancy. They will retreat to the deepest part of the pond and stay there until outside temperatures warm the water enough for them to maneuver around some.
Oxygen is important for the fish to survive, so either plan on the purchase of a pond surface heater (a horse tank heater will do) to maintain a hole in the ice. You may also place a pot with hot water on the ice surface to melt a hole as a temporary solution.
Ideally one should purchase a skimmer unit to incorporate into and on the pond’s surface along with a biological filter to help maintain balance in the water quality.
Typically spring will bring an early “bloom” to a pond’s water quality until filters and nature will eventually balance it and bring clarity.
If Koi and other fancy fish are your wish, restrict the number introduced into the pond by what the pond can support. In other words, don’t overwhelm your pond with too many fish, you’ll only be asking for trouble. Fish need oxygen to survive and too many will only restrict their growth and cause an imbalance in water quality because of the extra waste produced. Goldfish are a great example because they are dirtier than most fish and more prolific. I wouldn’t recommend them if clear water quality is what you desire.
On the other hand, Koi and Shebumpkins will give you the pleasant colors that are desired and will re-produce sparingly. In fact, larger fish will feed on the smaller ones so control is managed naturally. Butterfly tail Shebumpkins, with their multi-colored bodies, are especially pleasing.
All ponds, especially those with fish, require oxygenators to maintain water quality, so plan on purchasing plants for the water’s surface like water lettuce, water hyacinth and water lilies. Submersible plants are also recommended. Some will suspend themselves below the surface and some must be potted and placed on underwater shelves like the umbrella plant, Taro, and cattails.