The threadfish rainbowfish, also called the featherfin rainbowfish, are well known and cherished for the long flowing fins of the male. These are community schooling fish and best kept in a small school of six or more. AquariumFishSale.com will deliver in an even number of males and females. Males will display their pretty fins better in the presence of other tankmates. Females are easy to distinguish from the males, they do not have the long dorsal and anal fins.