One-lined Pencilfish, Nannostomus unifasciatus, is a small South American characin native to the Amazon and Orinoco river basins, with a natural distribution spanning Peru, Brazil, Colombia, Venezuela, and Guyana. It inhabits slow-moving tributaries, forest streams, flooded margins, and backwaters, often in areas with dense marginal vegetation, submerged roots, and leaf litter. These habitats are typically calm, warm, and lightly stained, with subdued lighting created by overhanging plants and forest canopy. Adult One-lined Pencilfish usually reaches about 1.5 to 2 inches in length. The body is long, slender, and torpedo-shaped, with a pale golden to silvery base color marked by a single, bold dark horizontal stripe running from the snout through the eye and along the length of the body. Depending on mood and lighting, this stripe can intensify or fade, and subtle reddish or amber tones may appear on the fins. Their streamlined shape and horizontal posture give them a delicate, elegant appearance that stands out in planted aquariums. A tank of at least 15 to 20 gallons is suitable for a proper group, with larger tanks always beneficial. One-lined Pencilfish are schooling fish and should be kept in groups of eight or more to display natural behavior and reduce stress. The aquarium should be densely planted, with floating plants and shaded areas to diffuse light, while still providing open space near the surface where they prefer to swim. Water flow should be gentle, as this species comes from calm or slow-moving waters. Water conditions should reflect their natural Amazonian environment. Ideal temperatures range from 74 to 80°F, with a pH between 5.5 and 7.0 and soft to moderately soft water. Stability and cleanliness are more important than exact numbers. Regular water changes and low nitrate levels help maintain health, coloration, and calm behavior. Feeding One-lined Pencilfish is easy but requires appropriately sized foods. In nature they feed on tiny insects, microcrustaceans, and other small prey taken from the water surface and midwater. In the aquarium they readily accept fine flakes, micro-pellets, and frozen foods such as baby brine shrimp, daphnia, and cyclops. They are gentle feeders and may be outcompeted by faster or more aggressive tankmates. Temperament is peaceful and reserved. One-lined Pencilfish do best with other calm, small species that share similar water requirements. Suitable tankmates include small tetras, rasboras, dwarf cichlids, Corydoras, and other peaceful fish. They are not well suited to boisterous or aggressive community tanks, as excessive activity can cause them to hide and lose coloration. Breeding is possible in captivity but not commonly achieved without targeted effort. They are egg scatterers that deposit adhesive eggs among fine-leaved plants or spawning mops. Adults do not provide parental care and may consume eggs if given the opportunity. Soft, slightly acidic water and subdued lighting increase the chances of successful spawning. Nannostomus unifasciatus is a subtle, graceful schooling fish that brings refinement and natural movement to planted aquariums. When kept in proper groups under calm conditions, the One-lined Pencilfish displays elegant behavior and understated beauty that rewards careful, attentive aquarists.
