Description
The one-lined pencilfish is spread over the upper Amazon and upper Orinoco, from Brazil, Bolivia and Colombia to Venezuela, and an introduced population lives in Trinidad and Tobago. It reaches about 4 cm and, like Nannostomus eques, swims tilted. To tell them apart: this one has no brown spots on its scales and usually shows an ocellus on the upper lobe of the tail.
The sexes are recognised by colour and anal fin: the male is showier, slimmer and has the larger anal fin.
Behavior
Very peaceful and shy, ill-suited to a lively community. Best in a group of ten or more, with which it gains confidence; there is barely any friction between males.
It lives well with peaceful characids of its size, corydoras and small loricariids, and works as a dither fish for dwarf cichlids because it moves in the middle and upper zones.
Avoid housing it with large, boisterous or very active fish.
Habitat
Nannostomus unifasciatus haunts slow creeks, minor rivers and marshy ground with dense vegetation and sunken wood. In the Caño La Guardia, on the llanos of Apure in Venezuela, it occupies a blackwater stream whose plain floods every year from May to October.
Floating plants, a dark substrate and a long tank suit it: it asks for a 75 cm base.
Pez lápiz de una línea is a schooling species, which means that in the wild it lives and moves in large, coordinated groups.
Keep at least 6 individuals so they feel comfortable and their stress level stays low.
Details
🌊 Water parameters
🐠 Minimum tank size
Breeding
No captive breeding is on record: the earlier reports appear to refer to Nannostomus eques, which lays its eggs under plant leaves.
Diet
A micropredator of tiny invertebrates.
- Staple: small-sized dry food.
- Supplement: brine shrimp or grindal worm, live or frozen, daily.
What about your tank?
Enter your tank's values to see only the compatible species that fit them. Litres apply to fish.
Compatible fish
General guidance based on water parameters, behaviour and known exceptions; always review the individual animals' and aquarium's specific needs.
They share water: pH 6–7 and 24–28 °C.
They share water: pH 7 and 26–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 6–7 and 23–27 °C.
They share water: pH 6–7 and 23–27 °C.
They share water: pH 6–7 and 23–27 °C.
They share water: pH 6–7 and 24–28 °C.
They share water: pH 5.5–7 and 24–27 °C.
They share water: pH 6–7 and 23–26 °C.
They share water: pH 5.5–7 and 23–28 °C.
They share water: pH 6–7 and 24–28 °C.
They share water: pH 6–7 and 25–28 °C.
They share water: pH 6–7 and 24–28 °C.
They share water: pH 6.5–7 and 24–26 °C.
They share water: pH 6–7 and 23–26 °C.
They share water: pH 6–7 and 23–25 °C.
They share water: pH 6–7 and 24–28 °C.
They share water: pH 6.5–7 and 23–24 °C.
They share water: pH 5–7 and 27–28 °C.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 6.5–7 and 23–28 °C.
They share water: pH 6–7 and 25–28 °C.
They share water: pH 6–7 and 23–28 °C.
They share water: pH 6–7 and 23–28 °C.
They share water: pH 6–7 and 24–28 °C.
They share water: pH 6–7 and 24–27 °C.
They share water: pH 5.5–7 and 24–28 °C.
They share water: pH 5.5–7 and 23–26 °C.
They share water: pH 4–6.5 and 23–27 °C.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 6–7 and 23–26 °C.
They share water: pH 6.5–7 and 23–24 °C.
They share water: pH 4.5–7 and 23–28 °C.
They share water: pH 5–7 and 23–27 °C.
They share water: pH 4–7 and 23–28 °C.
They share water: pH 7 and 23–27 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 5.5–7 and 26–28 °C.
They share water: pH 5.5–7 and 23–26 °C.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 4.5–6.5 and 24–28 °C.
They share water: pH 5.5–7 and 23–28 °C.
They share water: pH 5–7 and 23–27 °C.
They share water: pH 5.5–7 and 23–28 °C.
They share water: pH 5.5–7 and 24–28 °C.
They share water: pH 6–7 and 23–26 °C.
They share water: pH 5.5–7 and 23–27 °C.
They share water: pH 5.5–7 and 23–27 °C.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
Compatible invertebrates
General guidance based on water parameters, behaviour and known exceptions; always review the individual animals' and aquarium's specific needs.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 5.5–6.5 and 23–24 °C.
They share water: pH 6.5–7 and 23–27 °C.
They share water: pH 7 and 23–25 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 5.8–6.8 and 23–24 °C.
They share water: pH 5.8–6.8 and 23–24 °C.
They share water: pH 6.5–7 and 23–27 °C.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 6.5–7 and 23–26 °C.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
They share water: pH 6.5–7 and 23–28 °C.
They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
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They share water: pH 7 and 23–28 °C. They only meet at the very edge, so the tank has to hold that value.
Sources
- FishBaseFroese, R. and D. Pauly. Editors. 2026. FishBase. World Wide Web electronic publication.
- Seriously FishSeriously Fish. Nannostomus unifasciatus. Ficha de especie; consultada 2026-10.
Water: FishBase gives pH 5.5-7, up to 4 dH and 25-28 °C; Seriously Fish, pH 4-7, 1-10 dGH and 23-28 °C. The profile uses the widest range. Size: 3.8 cm SL in FishBase and 4 cm SL in Seriously Fish; the larger is taken.
Litres per fish and minimum tank height and depth: the profile's own judgement. Seriously Fish gives only the tank base and FishBase gives no tank sizes.
Plan your next aquarium
Is this fish right for you? Plan a suitable aquarium before setting it up.