The Tosa-Kin or curly fantail goldfish is a very distinctive breed of goldfish with a large tail fin that spreads out horizontally (like a fan) behind the fish. Though technically a divided tail, the two halves are attached at the center/middle forming a single fin.
It was developed in Japan, and is rarely seen in other countries. It is believed to have originally been developed from the ryukins.
Although it has a body shaped similar that of other fantailed goldfish, its tail fin opens and spreads flat and wide horizontally with the leading edges flipping under once or even twice. Because of its large and broad tail, the Tosakin is a weak and even clumsy swimmer requiring it be kept in still water without a strong current. The Tosakin has been often and mistakenly reported by as a weak fish due to inbreeding, however in reality is as hardy as most other breeds of fancy, deep-bodied goldfish. Like many other deep-bodied goldfish, the Tosakin is especially prone to swim bladder problems.
The Tosakin is considered a "top-view" fish and is traditionally kept in large shallow bowls or small ponds to be viewed from above. Seen from above, with its pointed head and deep, round trunk, the tail is obviously a flat half circle. Tosakins are metallic red, red and white or "uncolored" iron-black. Recently calico fish with Tosakin body conformations have been developed, but many purists have yet to recognize this coloration as a true Tosakin. The Tosakin is slow growing compared to most other breeds of fancy goldfish with a maximum attainable length of around eight inches.
The Tosa-Kin were first developed by a lower-ranking samurai in the Tosa fief (now Kochi Prefecture), in Shikoku, Japan and probably did not reach other parts of Japan until the mid-twentieth century.
U.S. air attacks on Kochi during World War II in 1945 and an earthquake in 1946 were believed to have wiped out the Tosakin variety. However, Mr. Tamura Hiroe, a Japanese hobbyist who had lost all of his fish, scoured the area and found six fish (2 breeders and 4 two-year olds) at a local restaurant called Kyousuirou. He managed to trade a large bottle of shouchuu for the fish, and was able to successfully revive the Tosa-Kin in Kochi. In 1969, the Japanese government declared the Tosa-Kin a Natural Treasure of Kochi Prefecture. In 1971 the breed had reached Tokyo, where a small group of young and avid enthusiasts painstakingly propagated and popularized the variety. The Tosa-Kin Preservation Society was founded a few years later in Aichi prefecture.