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Encyclopedia / Tetras / One-lined pencilfish
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Variations

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One-lined pencilfish

Nannostomus unifasciatus · One-striped pencilfish, Unifasciatus

Peaceful top and mid-water schooler.

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4 cmBody length, without tail ✓
24–28 °CTemperature ✓
5–7pH ✓
68 LSmallest tank ✓
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Tank mates
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Not recommended
  • Tinfoil barb eats animals up to about 5 cm. One-lined pencilfish only grows to 4 cm.
Not recommended
  • White cloud mountain minnow needs cool water and makes a lot of waste, while One-lined pencilfish is tropical.
  • They need different temperatures (24–28 °C vs 14–22 °C). No setting suits both.
Not recommended
  • Giant gourami eats animals up to about 10 cm. One-lined pencilfish only grows to 4 cm.
With conditions
  • Only a narrow temperature range suits both: 24–25 °C.
With conditions
  • Only a narrow temperature range suits both: exactly 24 °C.
Compatible
Compatible
Compatible
Compatible
Compatible

273 species in our list are a good match.

Facts checked against sources on 16 Oct 2026. ✓ means a source backs it; ? means no reliable source yet. See sources

Care and water

Hardness ✓1–10 dGH
Group size ✓10 or more
Temperament ✓Peaceful
Swims at ✓Top
Care level ✓Easy
Diet Omnivore
Lifespan ?Unknown
Size in shops 3–4 cm
Growth Medium

Behavior

Peaceful top and mid-water schooler.

Food

Small live and frozen foods such as baby brine shrimp and daphnia, plus fine flakes.

Tap a food to see what it is.

Male or female

Adult males are more colourful and usually slimmer than females and have an enlarged anal fin.

Health

Common problems. Tap one to see the signs. This is a guide, not a diagnosis.

Breeding

Not thought to have been bred in captivity; earlier breeding reports appear to refer to N. eques, which lays its eggs on the underside of plant leaves.

Full breeding guide, day-by-day plan and simulator →

Where it lives in the wild

RegionSouth America – Amazon & Orinoco basins and the Guianas
Rivers and lakesAmazon, Orinoco and Guiana rivers
HabitatSlow, vegetated waters
Native rangeAbout 1,500 km across
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The story of the one-lined pencilfish

A slender pencilfish with a single dark lengthwise stripe, usually a spot (ocellus) in the upper tail lobe, that swims at an oblique, head-up angle - a trait it shares only with N. eques.

Franz Steindachner described this pencilfish in 1876. It swims at an oblique, head-up angle, often hanging near the surface. A single black stripe runs from its snout into the lower tail lobe.

First described by Franz Steindachner in 1876. Named for 'unifasciatus' means single-banded.

  • It swims at an oblique, head-up angle.
  • Its single black stripe extends into the tail.
  • It feeds on tiny invertebrates near the surface.

Sources

16 facts checked with sources