One-Lined Pencilfish - Nannostomus Unifasciatus : Complete Fish Profile & Care Guide

Introduction

The One-Lined Pencilfish (Nannostomus Unifasciatus) is a captivating and esteemed species in the realm of freshwater aquariums. Native to the Amazon Basin in South America, this species is distinguished by its slender body, striking colouration, and captivating behaviour. With a long history of popularity among aquarium enthusiasts, the One-Lined Pencilfish has been admired and kept in captivity for decades. Renowned for its peaceful nature and intriguing social dynamics, this species is well-suited for community tanks when provided with suitable tankmates and a well-maintained aquatic environment that replicates its natural habitat.

One-Lined Pencilfish are renowned for their peaceful and social nature, often forming groups consisting of 10 or more individuals of the same species. While they can exist in solitary conditions, solitary individuals tend to display more timid behaviour, avoiding interactions with other species and seeking refuge among aquatic vegetation. Notably, males within the species exhibit a higher degree of aggression and territoriality, occasionally nipping the tails of other fish. Despite this behaviour, One-Lined Pencilfish is considered hardy and is compatible with a community tank comprising other non-aggressive fish.

Creating an ideal habitat for One-Lined Pencilfish entails the provision of a mature and spacious tank with subdued lighting. They thrive in a planted tank featuring floating plants that serve to diffuse the light, balanced with dense vegetation and open swimming spaces. These Pencilfish have a preference for a dark substrate over a lighter one. Maintaining water chemistry within slightly acidic conditions is essential for their well-being. Decorative elements such as driftwood branches and dried leaf litter contribute to a natural aesthetic within the tank.

In terms of their physical characteristics, One-Lined Pencilfish boasts a slender body ranging from pale to olive brown, complemented by a whitish belly and a small terminal mouth, complete with an adipose fin. A defining feature is a single black longitudinal stripe that extends the length of their body and onto the caudal fin, bordered above by a delicate metallic gold band. The caudal fin exhibits varying colouration depending on the geographic population, often featuring red or orange pigmentation in the lower quadrant, ventral to the black stripe.

In some particularly vibrant populations, the red pigment extends above the black stripe. Notably, the Guyanese population and a specific Bolivian form showcase a well-defined caudal ocellus above the black stripe. The ventral and anal fins are typically tipped with an enamel blue-white hue. An intriguing aspect of One-Lined Pencilfish is their distinct daytime and nighttime colour patterns; during darkness, the horizontal lines transform into a series of blotches extending from their back to their belly.