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 paradise fish is classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Perciformes, family Osphronemidae.