Nannostomus, the pencilfishes, is a genus of small, slender characins in the family Lebiasinidae, native to the Amazon basin, the Guyana shield, and adjacent river systems of tropical South America. The atlas's records span Brazil, Guyana, Peru, Venezuela, Colombia, Suriname, French Guiana, and Bolivia. True to their common name, pencilfish have a narrow, torpedo-shaped body, usually marked with one or more dark horizontal stripes, and swim in a distinctly head-up posture near the surface among dense vegetation in still, often acidic blackwater habitats. Many species are also known for a striking day-night color change, taking on a blotchy, broken pattern quite different from their daytime stripe when settled for the night. The atlas covers 20 species, among the smallest fish in the region, with adult sizes ranging from about 1.6 to just 2 in.
Pencilfish are longstanding favorites in the planted-aquarium and nano-tank side of the hobby, valued for their peaceful, unobtrusive schooling behavior and slim profile that suits densely planted setups. Nannostomus beckfordi (Golden Pencilfish) and N. marginatus (Dwarf pencilfish) are the most widely available species, while N. mortenthaleri (Coral Red Pencilfish), a comparatively recent addition to the trade, became an instant favorite for its vivid red coloration. Most species do best in soft, slightly acidic water reflecting their blackwater origins, and are generally undemanding once that basic water chemistry is met. Conservation status is mixed but not yet concerning for most of the genus: assessed species split fairly evenly between Least Concern and unassessed, with one species currently rated Critically Endangered.