What is "pencil fish"

Nannostomus (from the Greek: nanno = small, stomus = mouth) is a genus of fish belonging to the Lebiasinidae family of characins. All of the species in this genus are known as pencil fish or pencilfish, a popular name that was first only applied to two species in the 1920s, Nannostomus unifasciatus and Nannostomus eques, but by the late 1950s would come to be applied to all members of the genus. Several of the species have become popular aquarium fish due to their attractive coloration, unique shape, and interesting demeanor.