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With their flashing colors and interesting patterns, koi and goldfish are popular fish for aquariums and outdoor ponds. Among the two, goldfish are the older domesticated fish. The Chinese bred the first goldfish over 1,000 years ago. The Japanese bred koi from freshwater carp and introduced the first koi to the public in 1914. Today, both koi and goldfish are popular in Asia, the United States, and in many other countries worldwide.
Koi and goldfish require similar habitats. They require fresh water, and are raised in captivity in shallow ponds or aquariums. Both species require temperatures around 65 to 75 degrees, although they can tolerate slightly warmer or cooler temperatures. Water pH should be close to 7. Koi and goldfish need clean water, so an aquarium with a good filtration system is essential to remove waste buildup from the water.
Goldfish and koi are omnivores, meaning they eat both plants and insects. Both koi and goldfish enjoy duckweed (Leman minor), an invasive pond species. They're often added to ponds to help control duckweed. They will also eat algae, aquatic worms, and vegetable scraps such as finely diced raw peas and carrots. A commercial pellet or flake fish food offers a convenience basic diet for both koi and goldfish.
Koi and goldfish spawn in the spring when water temperatures reach around 68 degrees Fahrenheit. Females release eggs, and several males fertilize the eggs as they are released. Koi and goldfish eat their own eggs if they see them. In the wild, the eggs float to the shallow edges of the pond or lake and attach to water plants, where they remain hidden until hatching. In aquariums, spawning mats offer similar protection.
Disease affecting both types of fish include diseases caused by bacteria, virus, and protozoa. "Goldfish ulcer disease" and "koi ulcer disease" strike both fish, and are caused by the same bacterium, Aeromonas salmonicida. Overcrowding, stress, unsanitary conditions and a poor or monotonous diet make both koi and goldfish more susceptible to illness.
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