Comets, or comet-tailed goldfish, are one of the most popular types of fancy goldfish in the United States. They're similar to common goldfish, except comets are slightly slimmer and smaller with long, forked tails. Comets are very hardy, easy to care for and are more active than most other goldfish.
Housing Comets with Other Fish
Comets can be housed with other fish, but you want to be careful when deciding which fish to put in a tank with them. Like most goldfish, comets can grow large, up to 13 inches, so even for a small group of comets, you want at least a 55-gallon aquarium -- a 75-gallon is preferred. These fish require cool water, so it's not a good idea to put tropical fish in an aquarium with them. Goldfish are also messy fish, so you want to keep an eye on the ammonia, nitrite and nitrate levels; as long as the aquarium is is adequate, the water should dilute the excess waste on its own (also given regular water changes occur).
Not all fish make good tank makes for comets, but you can choose from most varieties of goldfish. Black moor goldfish, fantail, shubunkin (calico), pearlscale and common goldfish are all hardy species that can thrive in colder water, just like comets. They can bring variety into your large goldfish aquarium. Not all goldfish will thrive with comets; avoid housing them with bubble-eye, celestial eye or telescope goldfish because they are more delicate goldfish varieties.
If you do not want a tank full of goldfish varieties, you may want to consider gold barbs, dojo loaches, guppies and zebra danios. Barbs and danios can be nippy with tank mates and do best in small groups of at least six fish, but you may find that they still may nip on the goldfish's fins. Pleco bristlenose can be housed with comet goldfish, too; they can serve as algae-eating fish, cleaning up the tank debris, as well as just a basic tank mate to your comets.
Snails and Invertebrates
Mystery snails and apple snails can be housed with comet-tail goldfish. For the most part, the snails will stay out of the way, making them less noticeable to the fish. Most aquarium invertebrates require warmer water, so you want to be careful of placing them in the aquarium with your comets. Plus, keep in mind that comets are opportunistic feeders and may try to eat any small invertebrates and crustaceans that you house with them.