One-lined Pencilfish

About

The one-lined pencilfish (Nannostomus unifasciatus) is a slender, small characid from across a surprisingly wide stretch of South America, turning up in parts of the Amazon basin through Bolivia, Brazil, and likely Colombia, as well as the upper Orinoco drainage in Venezuela and Guyana. Its most immediately noticeable trait is its swimming posture: the fish tilts head-up at an oblique angle more or less constantly, a quirk it shares only with its close relative N. eques among pencilfish. Identification is straightforward once you know what to look for. A single bold lateral stripe running the length of the body, a small ocellus in the upper caudal lobe, and clean body scales without dark spotting (which rules out N. eques). Like most of its genus, it switches to a cryptic, vertically-barred pattern after dark, an automatic rhythm that occurs even in blind individuals. In the aquarium it's a peaceful, somewhat timid fish that genuinely needs companions of its own kind. Groups of ten or more bring out bolder, more natural behavior and reduce any low-level squabbling between males. It does best alongside small, peaceful characids or diminutive catfishes, and works well as a dither fish in setups housing Apistogramma or similar dwarf cichlids. It has not been reliably bred in captivity. A rewarding fish for patient intermediate keepers with the right tank setup.