One-Lined Pencilfish

Nannostomus unifasciatus

Also known as One-Stripe Pencilfish, Unifasciatus Pencilfish

Animalia›Chordata›Characiformes›Lebiasinidae

The One-Lined Pencilfish is a slender, peaceful nano fish native to South American blackwater streams, characterized by a single dark horizontal stripe running the length of its body. These tiny fish are excellent for planted nano tanks and community setups, displaying graceful, deliberate swimming patterns. They are best kept in small schools and thrive in soft, acidic water conditions.

Where it's from

Native to the Amazon, Demerara, Essequibo and Orinoco river basins

Brazil, Bolivia, Guyana, Colombia and Venezuela

Recorded as introduced in Trinidad & Tobago

At a glance

Size: 1.4 in; 4 cm
Min tank: 10 gal; 38 L
Group: 6+
Temp: 72–79°F; 22–26°C
pH: 5.5-7.0
GH: 36–143 ppm; 2–8 dGH
Temperament: Peaceful
Swims: Top
Lifespan: 3-5 yrs

Ammonia 0 ppm and nitrate under 20 ppm.

Care

Diet

One-Lined Pencilfish are micropredators that feed on small live foods such as infusoria, micro worms, and newly hatched brine shrimp. Supplement with high-quality micro pellets and finely crushed flake food. Feed small amounts 2-3 times daily, offering only what they can consume in a few minutes.

Behavior

These fish are shy, peaceful schoolers that spend most of their time in the middle water column, often hovering near plants. They exhibit a characteristic head-down posture when at rest and are most active during low-light periods. They should never be kept singly, as they become stressed and hide constantly without companions.

Breeding

One-Lined Pencilfish are egg scatterers that breed readily in well-established, heavily planted tanks with soft, acidic water. Breeding pairs will scatter eggs among fine-leaved plants; remove adults after spawning to prevent egg predation. Fry are extremely small and require infusoria or liquid fry food for the first week.

Tank mates

See all 6

Neon Tetra

Similar size, peaceful temperament, and water parameter requirements

excellent

Ember Tetra

Comparable nano fish with matching care needs and peaceful behavior

excellent

Otocinclus

Peaceful bottom dweller that won't compete for food or space

excellent

Dwarf Hairgrass Mini

Provides shelter and natural environment without competing

excellent

Amano Shrimp

Peaceful invertebrate that occupies different ecological niche

good

Fits a 10-gallon tank Peaceful

No state or federal restrictions on record.

Not legal advice, and possibly incomplete or out of date. Rules vary by state and change over time; check with your state wildlife or agriculture agency before buying, keeping or shipping it.

The care information here is written with AI help, so it can contain mistakes. Spot one? Tell us and we'll fix it. Native range: Catálogo Taxonômico da Fauna do Brasil (CC BY 4.0); Global Register of Introduced and Invasive Species (CC BY 4.0); Lista Roja de la Fauna Venezolana 2015 (CC BY 4.0); Tedesco et al. 2017 (CC0).