Honey Red Pencilfish (Nannostomus rubrocaudatus)

The Coral Red Pencilfish (Nannostomus rubrocaudatus) — commonly traded as Honey Red or Purple Pencilfish — is native to the blackwater streams of the Ucayali and Nanay river systems in Peru's upper Amazon basin. Formally described in 2008, it represents one of the most recently described and most strikingly beautiful of the Nannostomus pencilfish species.

Males are extraordinarily colored for such a small fish (3–4 cm): the upper body is pale yellow-tan to silvery, traversed by a vivid red-orange to crimson horizontal stripe from the snout to the caudal fin base. The fins — particularly the dorsal, anal, and pelvic fins — are brilliant crimson to coral-red, and the caudal fin is deeply red-pigmented. In optimal condition under appropriate lighting, males appear to glow with internal fire. Females are less vivid but still show attractive red fin coloration.

In the wild, these pencilfish inhabit soft, acidic, tannin-stained blackwater streams with dense fine-leaved vegetation and leaf litter. Groups of males engage in continuous, formalized lateral displays where they hover side-by-side, spreading fins and vibrating to assert dominance and court females — behavior that is endlessly fascinating in the aquarium.

Pencilfish characteristically adopt a head-up angled posture at rest, a trait shared across the Nannostomus genus that makes them immediately recognizable and distinctive in the aquarium.

Honey Red Pencilfish require soft, acidic blackwater conditions for optimal health and coloration: pH 4.5–6.5, hardness 1–6 dGH, and temperatures 24–28°C. Soft, tannin-rich water (driftwood, Indian almond leaves, dried botanicals) is highly beneficial for maintaining the most vivid coloration and natural behavior.

A minimum 40-liter blackwater planted aquarium is suitable for a group of 8–10. Provide a dark substrate, driftwood, dense fine-leaved plants (Java moss, Rotala, Staurogyne), and floating plants for diffused lighting. Gentle filtration — sponge filters or spray bars at low flow — replicates their still to gently flowing natural habitat.

Feed appropriately tiny foods: nano pellets, baby brine shrimp nauplii, daphnia, micro worms, and cyclops. Their mouths are small — ensure food particle size matches. Feed small amounts 2–3 times daily.

Honey Red Pencilfish are peaceful and suitable with other small, gentle blackwater species: small Corydoras, Boraras rasboras, small Apistogramma, and small shrimp (though males may occasionally display at passing shrimp). Avoid larger fish or anything nippy.

Keep in groups of at least 8, with more males than females encouraged to maximize display behavior. Males are non-injurious in their displays — higher numbers intensify fascinating behavioral complexity. Regular small water changes (20% twice weekly) with soft water maintain conditions.