Koi
Pond carp bred for colour, and long lived with it: decades is normal. Koi learn who feeds them and will take food from a hand.
Overview
Koi are ornamental carp bred for colour and pattern, and they are pond fish rather than aquarium fish. UF extension material puts adult size at over 600 mm, about 24 inches, and koi keepers describe 24 to 30 inches in a warm, well fed, clean pond.
They are also a long commitment. Well cared for they reach around 30 years, while the same source notes that with little care they may manage only 5 to 7. The difference is filtration, volume and water quality rather than anything about the fish.
Temperament
Calm, curious and unusually willing to be handled by pond standards. Koi learn to take food from a hand and to come when someone approaches the edge.
They are social and are kept in groups of five or more. A koi that isolates itself from the group is usually the first visible sign that something is wrong, which is why keepers watch the shape of the group as much as the individual fish.
Care and grooming
They forage off the bottom for mosquito larvae, crustaceans and whatever else is in the pond, and they take manufactured pellets readily, preferring sinking food to floating.
Water quality is the whole job. A published review gives an optimum of 23 to 25 degrees C, about 73 to 77 F, a pH optimum of 7.0 to 7.5, and ammonia optimally below 0.025 mg/L with anything above 0.3 mg/L critical. In winter the fish go torpid as the water cools and eat very little until spring, so feeding follows temperature rather than the calendar.
Living and exercise
A pond, filtered heavily, with a few hundred gallons per fish as a floor and many keepers allowing 1,000 gallons each. Some of the water should be at least four feet deep so the fish have somewhere stable to sit through winter.
The list of things that eat koi is long: herons, kingfishers, raccoons, cats, turtles and rats among them. Depth, cover and netting are the usual answers, and a shallow ornamental pond in an open garden is the arrangement that loses fish.
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