noun
comet goldfish
A variety of goldfish with a long, flowing tail fin, resembling a comet.
テツギョは尾びれが長くて美しい。
The comet goldfish has a long, beautiful tail fin.
From 鉄 (tetsu, 'iron') + 魚 (gyo, 'fish'), possibly referring to the metallic sheen or the iron-like color of some varieties. The exact derivation is uncertain.