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The meteor goldfish is considered the rarest breed of goldfish. They are a tailless form thought to have been developed by goldfish breeders during the late 19th or early 20th century. The meteor goldfish lack a tail fin, but have a well-developed anal fin in its position. The other fins of the meteor goldfish are elongated, and it is a competent swimmer despite its lack of a tail. Few swim like other normal goldfish and few swim straight ahead like a rocket, the rocket swimming being rarer. The meteor is also one of the hardest to breed, as they can be very weak, and in low temperatures sicken easily.
References
Andrews, Dr. Chris. "An Interpet Guide to Fancy Goldfish", Interpet Publishing, 2002 - ISBN1-902389-64-6