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No federal permit to keepOneline pencilfish
Nannostomus unifasciatus
Fishes › Tetras, piranhas and pacus (Characiformes) › Lebiasinidae (Pencilfish and splash tetras)
Nannostomus unifasciatus (from the Greek nanos, meaning 'small', and the Latin stomus, relating to the mouth; the Latin unifasciatus = 'one band'), commonly known as the one-lined pencilfish, is a freshwater species of fish belonging to the genus…
Care at a glance
Species-specific aquarium care
- Tank: At least 24 in long and 12 in front to back for one adult (adults commonly reach about 1.5 in / 3.8 cm).
- Water: Freshwater.
- Temperature: 77–82°F (range recorded for this species).
- pH: 5.5–7.
- Hardness: 0–4 dGH.
- Layout: Uses both the bottom and open water: give open swimming space plus cover near the substrate.
- Diet: A predator: feed meaty foods sized to its mouth (frozen fish, shrimp, worms); it will eat tankmates that fit in its mouth. FishBase classes it as a carnivore.
Calculated from this species' own FishBase record: tank size from its typical adult length and swimming zone, water values from its recorded ranges. The group guide below fills in general care.
Care guide
Species care notes
- Tank: a base of at least 30 × 12 in (roughly 18 US gallons); space needed is rated “modest tank”.
- Water: soft, acidic water, 73–82 °F, pH 4–7, 1–10 dGH.
- Behaviour: temperament peaceful; a shoaling fish best kept in a group of six or more; suited to a community tank.
- Difficulty: Easy (Seriously Fish rating).
Short excerpts from the species profile:
- Setup: “Should ideally be kept in a heavily-planted set-up, preferably with a dark substrate and some patches of floating vegetation around which the fish will tend to congregate, bodies angled towards the surface.”
- Diet: “A micropredator feeding on tiny invertebrates and other zooplankton in nature.”
- Tankmates: “Very peaceful but does not make an ideal community fish due to its small size and rather timid nature.”
- Breeding: “Not thought to have been bred in captivity, with earlier reports all appearing to pertain to the congener N.”
From the Seriously Fish profile for this species, which has the full habitat, maintenance, diet, behaviour and breeding write-up 167
Pencilfish and splash tetras
Applies to most species in this group. Habitat: Common pencilfish and splash tetras inhabit vegetated South American streams and tributaries, with Copella also entering flooded forest. Individual species can differ, so compare with the species numbers above and the linked sources.
Housing & setup
- N. beckfordi needs a base of at least 18 × 12 inches; C. arnoldi needs at least 36 × 12 inches for long-term care.
- Use dense planting, floating plants, branches and leaf litter for shade and broken sight lines.
- Use gentle filtration for pencilfish; provide well-oxygenated water with slight surface agitation for splash tetras.
- For breeding C. arnoldi provide about 4 inches between water and cover, plus overhanging spawning surfaces.
- Use a biologically mature aquarium; splash tetras are sensitive to chemistry swings.
Climate & lighting
| Water | 69.8–80.6°F |
Diet
- Dry foods small enough for their mouths.
- Daily small live or frozen Artemia nauplii, Moina or grindal worms.
- Small gut-loaded fruit flies or crickets for C. arnoldi.
Social life
- Keep N. beckfordi in groups of 10 or more and C. arnoldi in mixed-sex groups of 8–10; choose peaceful small companions. Male pencilfish spar, so provide visual barriers.
Health watch
- Water-quality stress in immature aquaria.
- Nutritional deficiencies from unsuitable food sizes or limited diets.
- Freshwater white spot and other skin/gill parasites.
Understanding the animal
About this animal
About this animal
They are popular in the aquarium trade due to their small size, beautiful color pattern, unique swimming posture and relative hardiness. With its long, pencil-shaped profile, its single jet black stripe reminiscent of the graphite core of a pencil, and its eraser-colored caudal fin, this is the species for which the popular name, 'pencilfish', was first coined in the 1920s, later to be applied to all the species of the genus Nannostomus.
Quoted from the Wikipedia article “Nannostomus unifasciatus” (CC BY-SA 4.0) 174
In the wild
In the wild
From this species’ own FishBase record (habitat, bottom type, water and breeding data), not its family. Good care copies these conditions.1
| Where it lives | Lives in freshwater. Uses both the bottom and open water. Feeds on worms, crustaceans and insects. |
| Water it lives in | Freshwater, 77–82 °F (25–28 °C), pH 5.5–7, hardness 4 dH. |
| Climate | Tropical climate. |
| Wild diet | Hunting macrofauna (predator). |
| Behavior to plan for | Breeding: open water/substratum egg scatterers. |
What this means for care
- Keep water within its wild range, 77–82 °F.
Native climate (measured)
Native climate (measured)
| Warm-season highs | about 91°F (up to 95°F in the hottest sampled areas) |
| Cool-season lows | about 71°F (down to 68°F in the coldest sampled areas) |
| Average temperature | 79–81°F across sampled records |
| Relative humidity | 78–84% on average (driest month about 76%) |
| Rainfall | about 2352 mm a year |
Measured at 233 wild occurrence records sampled against WorldClim 2.1 climate data (1970–2000 averages). Air conditions in the range are ecological context, not enclosure settings: microhabitats such as burrows, leaf litter or basking rocks can be cooler, warmer or more humid.
From the sources
From the sources
Quoted from the matching sections of reputable sources for this exact species. Specialist databases are used first; Wikipedia (community-edited, CC BY-SA 4.0) only fills gaps. Other sources are short credited quotes.
| Overview | Nannostomus unifasciatus (from the Greek nanos, and the Latin stomus, relating to the mouth; the Latin unifasciatus = ), commonly known as the one-lined pencilfish, is a freshwater species of fish belonging to the genus Nannostomus in the characin family Lebiasinidae. They are popular in the aquarium trade due to their small size, beautiful color pattern, unique swimming posture and relative hardiness. — Wikipedia (community-edited) |
| Where it lives | Home ranges Population dynamics Growth parameters Max. ages / sizes Length-weight rel. Length-frequencies Mass conversion Recruitment Abundance Life cycle Reproduction Maturity Maturity/Gills rel. — FishBase |
| Wild diet | Nannostomus unifasciatus is omnivorous and feeds primarily on insects and small crustaceans. It is also an avid periphyton grazer, ingesting microorganisms as well as algae. — Wikipedia (community-edited) |
| Appearance and size | Though the one-lined pencilfish is a small fish, with adults not reaching more than 7 cm in length, it is nevertheless one of the largest species of Nannostomus. It is a long, slender species with a small terminal mouth and an adipose fin. It has one black longitudinal stripe that runs the length of the body and onto the caudal fin. — Wikipedia (community-edited) |
| Biology | Feeds on worms, crustaceans and insects. — FishBase |
| History and naming | Steindachner first described the species in 1876, making it the second of what are now nineteen species in the genus to be described. — Wikipedia (community-edited) |
Data and studies
The numbers
The numbers
Each bar is what one source reports. Where several sources weigh in, the dashed gold band marks their average.
Adult length
Maximum recorded length. Measured as standard length (snout to the base of the tail, not counting the tail fin). Fish in home aquariums often stay smaller than wild records.
Water temperature
Range the species tolerates in nature; aim for the middle.
Water hardness
Minimum tank footprint
Seriously Fish recommends a tank base of at least 30 × 12 in (about 18 US gallons).
Sources for this sheet
- FishBase (Froese & Pauly, eds.), version 24.07 Peer-reviewed researchThe global scientific database on fishes, maintained by a consortium of research institutions; every value is referenced.
- Seriously Fish species profiles Experienced keeper or breederLong-running aquarium reference written and peer-checked by experienced fishkeepers and ichthyologists; each profile covers habitat, tank size, water, diet, behaviour and breeding.
- Nannostomus beckfordi — Seriously Fish Experienced keeper or breederAdded in the 2026 source-checked care-guide review.
- Copella arnoldi — Seriously Fish Experienced keeper or breederAdded in the 2026 source-checked care-guide review.
- Disorders and Diseases of Fish — Merck Veterinary Manual Zoo, university or veterinaryAdded in the 2026 source-checked care-guide review.
- Parasitic Diseases of Fish — Merck Veterinary Manual Zoo, university or veterinaryAdded in the 2026 source-checked care-guide review.
- Wikipedia (species article) Community referenceFree encyclopedia; useful overviews with their own citations. Intro text is quoted under CC BY-SA 4.0.
More pencilfish and splash tetras
Barred pencilfish
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Brown pencilfish
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Copella carsevennensis
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Golden pencilfish
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Least pencilfish
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Nannostomus marginatus
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