Comet goldfishes are among the most prevalent of the fancy goldfish types. They look like regular goldfish with longer tail fins. Distinct from twin-tailed goldfish, comet goldfish have tail fins with two long lobes, split horizontally.
You can divide goldfish into two main types, fancy and common. Common goldfish have "standard" or wildtype fins. They can come in a wide variety of colors instead of just the drab silvery brown of their carp ancestors. Most feeder goldfish fall into this category, though some exceptions exist. Feeder goldfish are those bred to become food for other fish, instead of to live as pets. Generally, feeder goldfish are bred and kept under crowded conditions that leaves them sickly and potential disease carriers. They're short-lived and less-than-ideal pets -- though many people bring them home expecting them to thrive.
Fancy goldfish are bred to be flashier than the common goldfish in some way beyond coloration. Long fins, unusual body shapes and bizarre growths are some variations. Comet goldfish are among the simplest of the fancy goldfish. They are almost identical to common goldfish but have longer, flowing tails. Some confusion exists because some fancy goldfish breeding programs will use their culls -- fancy goldfish who do not possess whatever desired traits -- and use them as feeder fish or throw them into breeding pools with feeder fish. So plenty of feeder goldfish for sale have comet tails but are likely not true comet goldfish.
Comets as Pets
Most people don't realize that goldfish can grow to larger than a foot and can live longer than 40 years under ideal conditions -- so most goldfish rarely get adequate care. They need certain conditions met in order to thrive, but they're not picky like tropical fish. As a temperate species, goldfish in general can survive in unheated aquariums and even outdoor ponds. Make sure you have 30 gallons of aquarium or pond capacity per fish to avoid overcrowding.
Frequently sold as fish food, goldfish -- including comet goldfish -- provide very poor nutrition. They are rich enough in fat to cause fatty deposits to build up in the livers of other fish. Additionally, goldfish flesh contains an enzyme called thaiminase. This enzyme breaks down vitamin B, causing deficiencies in animals eating them. Vitamin B deficiencies can cause all kinds of problems, including nerve issues and additional liver problems. Feeding goldfish to other fish is a common practice, but it's not recommended.