Brown pencilfish
Nannostomus eques
The brown pencilfish (Nannostomus eques) is a small, peaceful South American shoaling fish with a distinctive oblique, head-up swimming posture. It is shy and has a tiny mouth, so a calm, well-structured aquarium and fine varied foods matter. Keep it preferably with ten or more conspecifics. Despite its modest size, it is not ideal for a busy, newly established or highly competitive community tank. Marginal shelter and an open feeding area help it feel secure without removing swimming space. Keep its food small enough for its mouth.
Plan your aquarium →At a glance
- Temperature
- 23–28 °C
- pH
- 5.0–6.0
- GH
- 5–10 °dGH
- Aquarium length
- At least 60 cm
- Minimum aquarium volume
- At least 54 L
- Adult size
- Approximately 3.5–5 cm
- Group size
- At least 10 individuals
- Temperament
- Peaceful
About this species
The accepted name is Nannostomus eques Steindachner, 1876. It belongs to Lebiasinidae in the order Characiformes. Older literature and hobby references also used the genera Poecilobrycon and Nannobrycon; these are historical combinations, not separate species.
The slender body and characteristic upward-angled posture make it distinctive. Seriously Fish reports 35 mm standard length, while FishBase gives a maximum of about 5 cm total length. These are different measurements: the caudal fin is included only in total length.
Natural habitat
FishBase records the species from the central and upper Amazon; specialist accounts also mention the Orinoco drainage and rivers in Guyana. Distribution accounts are not fully identical, so no single basin boundary is presented here as absolute.
It inhabits sluggish tributaries, small rivers, swampy areas and flooded forest. Dense marginal vegetation, submerged wood and leaf litter are typical. Some blackwater sites are extremely soft and acidic. Those are field conditions, not a requirement to make every aquarium artificially acidic.
Aquarium and layout
Seriously Fish specifies a minimum aquarium footprint of 60 × 30 cm, about 54 litres at a typical depth. FishBase also lists a minimum length of 60 cm. These are guidelines, not a guarantee that every stocking plan suits that volume. Ideally keep a shoal of at least ten; a larger group helps this timid fish feel secure.
Plant the margins densely with fine plants and wood, optionally with floating cover, while leaving calm swimming and feeding space. Use a mature filter and avoid a forceful jet. Keep the tank securely covered because startled fish may jump. Add them only after biological filtration is stable.
Water conditions
Seriously Fish gives broad species ranges of 22–28 °C, pH 4.5–7.5 and GH 1–10 dGH. FishBase lists 23–28 °C, pH 5–6 and hardness 5–12. A cautious shared range is therefore 23–28 °C, pH 5–6 and GH 5–10 dGH. Natural blackwater sites can be much softer and more acidic than this overlap.
Stability matters more than chasing an exact number. Do not change pH or hardness abruptly; match acclimation to the water in which the fish were kept. Reviewed sources do not establish a species-specific KH target.
Behaviour and group size
This is a very peaceful, small and timid fish. Preferably keep ten or more; larger groups are often more confident and visible. Choose calm tankmates of similar size. Small peaceful characins or small, calm corydoras may fit when their group needs, swimming zone and adult size also match.
Avoid large predators, boisterous feeders and species that outcompete pencilfish for food. Shared temperature or pH alone does not prove suitability. Check that every fish eats, is not constantly startled and has shelter. Separate fish if persistent stress or food exclusion occurs.
Feeding
In nature N. eques feeds on tiny invertebrates and zooplankton. Offer very small food particles. It may accept suitably sized dry food, but also provide small live or frozen foods daily, such as Artemia nauplii, Moina or grindal worms. Distribute small portions in calm areas so shy individuals can feed.
Observe actual food intake rather than assuming that any flake or pellet is suitable. Remove leftovers to protect water quality. A varied menu of appropriately small prey is more useful than treating one product as a complete diet.
Breeding
Breeding has been achieved in aquaria but is described as rare. Unlike many related pencilfish, this species deposits eggs in single batches on the undersides of broad leaves. Anubias, Java fern (Microsorum) and Echinodorus are cited as possible surfaces. Adults eat eggs they find; a separate mature breeding tank and timely removal of adults can reduce losses.
First foods must be extremely small, such as Paramecium or suitable fine fry food. Introduce Artemia nauplii or microworms only when fry can take them. Reported incubation times are not guaranteed for every aquarium. Keep lighting subdued and water stable.
Sources & review
This care guide draws on several specialist sources and was last checked on .