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Goldfish & Koi
Goldfish and Koi are, respectively, single species that are both in the family Cyprinidae. This is the same family that is shared by Loaches, Barbs, Freshwater Sharks, and Danios.
These two popular ornamental species were selectively bred in Asia over thousands of years to produce a variety of body shapes and brilliant color variations. There are now many recognized basic color varieties of Koi and many standard body shapes for Goldfish—none of which exist in nature. Over the past few decades the popularity of the Goldfish has decreased as hobby newcomers have realized the advantages that true tropical fishes have over this species. However, this has not diminished interest in fancy goldfish varieties, some of which are eagerly sought after and command high prices. Like Koi, Goldfish have a high temperature tolerance. They have been known to live outdoors in water beneath a layer of ice for quite a time (though this is not recommended) and can likewise withstand warm, tropical water.
Regardless of whether a Goldfish is kept in a small bowl or a Koi is kept in a large pond, these fishes require proper aeration. Goldfish will essentially eat any flakes or pelleted diets that are offered, provided they are not too big for the fish to swallow. Larger Koi will accept some frozen diets but are typically fed large pellets.