What species of cold-water ornamental fish are there?

Common cold-water ornamental fish include goldfish, koi, bettas, red devil fish, medaka, sunfish, and pike-perch. The easiest to keep are comet goldfish, koi, and Chinese bettas; comet goldfish are vigorous and adaptable, surviving even in low-oxygen water. Koi also have strong constitutions and varied patterns, giving high ornamental value. Finally, Chinese bettas are popular, small but undemanding about the environment, pretty yet easy to keep alive.