Goldfish
The goldfish ("Carassius auratus") is a freshwater fish in the family Cyprinidae of order Cypriniformes. It is one of the most commonly kept aquarium fish.
Comet (goldfish)
The comet or comet-tailed goldfish is a single-tailed goldfish bred in the United States. It is similar to the common goldfish, except slightly smaller and slimmer, and is mainly distinguished by its long deeply forked tail. 
Comet goldfish tend to have a diverse variety of colors, unlike the common house pet goldfish.