paradise fish, brilliantly colored freshwater Asian fish, Macropodus opercularis, often kept in aquariums. The males reach a length of 3 in. (7.6 cm) and turn reddish with blue bars during mating season. Fantastic varieties with greatly extended and modified fins and tails have been developed. The round-tailed paradise fish Macropodus chinensis is a close relative. Paradise fish are classified in the phylum Chordata, subphylum Vertebrata, class Osteichthyes, order Perciformes, family Belontiidae.