Dwarf Pencilfish (Nannostomus marginatus)

Nannostomus marginatus, the Dwarf Pencilfish, is a small and attractively patterned pencilfish from the Amazon and Orinoco River basins of South America. It is one of the most commonly kept pencilfish species, offering a balance of availability, visual attractiveness, and relative hardiness that makes it suitable for intermediate aquarists.

The body is elongated and slender in the classic pencilfish form, displaying several distinctive dark horizontal stripes on a pale cream to white base colour. In mature males, vivid red colouration appears in the dorsal, anal, and ventral fins, creating a striking contrast with the striped body pattern. This red fin colouration intensifies dramatically when males are competing — producing one of the most beautiful small-fish displays available.

Like all pencilfish, the Dwarf Pencilfish has the characteristic nocturnal resting behaviour where individuals tilt their bodies at approximately 45 degrees, head down, when at rest during the night. This is a normal species-specific behaviour that frequently alarms new keepers when first observed.

The Dwarf Pencilfish inhabits slowly flowing blackwater streams and floodplain pools in the Amazon and Orinoco basins, preferring very soft, slightly acidic water. In captivity, it adapts to a broader range of conditions than some other pencilfish but performs best in soft, slightly acidic water with dense planting.

Tank Setup: A 40-litre minimum for a group of eight. Dense planting with fine-leaved species (Java moss, Hornwort, Rotala), dark substrate, gentle filtration, and moderate to subdued lighting. Tannins from Indian almond leaves or peat enhance colour and replication of blackwater conditions.

Water Parameters: Temperature 23–28°C; pH 5.5–7.5; GH 2–12 °dH; KH 1–6 °dH. Soft to moderately soft, slightly acidic water for best colour expression. More adaptable than many pencilfish.

Feeding: Omnivore. Small mouths require micro foods: micro pellets, crushed flake, baby brine shrimp, Daphnia, and micro worms. Feed twice daily. Ensure food particle size is appropriately small.

Tank Mates: All peaceful nano and small community species: Ember Tetras, small Boraras, Pygmy Corydoras, Otocinclus, small Apistogramma, and peaceful dwarf shrimp. Avoid any aggressive or significantly larger fish.

Behaviour: Active during daylight, adopts head-down nocturnal resting posture. Groups of 8+ with more females than males show most natural display behaviour — males compete with vivid red fin displays. Peaceful and non-aggressive toward all other species.

Health: Moderately hardy for a pencilfish. Sensitive to elevated nitrates and sudden chemistry changes. Weekly 20% water changes and stable parameters maintain excellent health.