Home / Fishes / Tetras, piranhas and pacus / Nannostomus marginatus
No federal permit to keepNannostomus marginatus
Nannostomus marginatus
Fishes › Tetras, piranhas and pacus (Characiformes) › Lebiasinidae (Pencilfish and splash tetras)
Nannostomus marginatus (from the Greek: nanos = small, and the Latin stomus = relating to the mouth; from the Latin: marginatus = marginated), commonly known as the dwarf pencilfish, is a freshwater species of fish belonging to the genus Nannostomus…
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 1.4 in / 3.5 cm).
- Water: Freshwater.
- Temperature: 75–79°F (range recorded for this species).
- pH: 5.8–7.5.
- Hardness: 0–15 dGH.
- Layout: Uses both the bottom and open water: give open swimming space plus cover near the substrate.
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 18 × 12 in (roughly 11 US gallons); space needed is rated “modest tank”.
- Water: soft, acidic water, 72–82 °F, pH 4–7, 1–10 dGH.
- Behaviour: temperament peaceful; a shoaling fish best kept in a group of six or more; not a community fish.
- Difficulty: Moderate (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.”
- 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: “Has been achieved in aquaria, albeit rarely, and in a densely-planted, mature aquarium it's possible that small numbers of fry may start to appear without intervention.”
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
The most notable feature of N. marginatus is its size, being one of the smallest members of the genus, only reaching a maximum size of 35 mm (1.4 in).
Quoted from the Wikipedia article “Nannostomus marginatus” (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. Aquarium keeping: in groups of 5 or more individuals; minimum aquarium size 60 cm. |
| Water it lives in | Freshwater, 75–79 °F (24–26 °C), pH 5.8–7.5, hardness 15 dH. |
| Climate | Tropical climate. |
| Behavior to plan for | Breeding: open water/substratum egg scatterers. |
What this means for care
- Keep water within its wild range, 75–79 °F.
Native climate (measured)
Native climate (measured)
| Warm-season highs | about 90°F (up to 92°F in the hottest sampled areas) |
| Cool-season lows | about 71°F (down to 68°F in the coldest sampled areas) |
| Average temperature | 78–81°F across sampled records |
| Relative humidity | 79–85% on average (driest month about 78%) |
| Rainfall | about 2368 mm a year |
Measured at 189 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 marginatus (from the Greek: nanos = small, and the Latin stomus = relating to the mouth; from the Latin: marginatus = marginated), commonly known as the dwarf pencilfish, is a freshwater species of fish belonging to the genus Nannostomus in the characin family Lebiasinidae. They were first described in 1909 by C. H. Eigenmann, and are typical of members of this genus, being small, elongated fish with prominent horizontal stripes. The most notable feature of N. marginatus is its size, being one of the smallest members of the genus, only reaching a maximum size of 35 mm. — 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 |
| Biology | Aquarium keeping: in groups of 5 or more individuals; minimum aquarium size 60 cm. — FishBase |
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 total length. 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 18 × 12 in (about 11 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.
- iNaturalist (captive observations) Community referenceCommunity observation platform run by the California Academy of Sciences and National Geographic; observations marked captive show animals living in homes, zoos and aquariums.
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