Mortenthaleri Pencilfish

Nannostomus mortenthaleri, the Coral Red or Red Arc Pencilfish, originates from the Rio Nanay system in the Peruvian Amazon — a blackwater river of extraordinary biological diversity. This species regularly tops popularity polls among pencilfish enthusiasts due to its combination of small size, intense colouration, and intriguing social behaviour. The flanks burn in coral-red and orange hues, framed by a crisp dark lateral stripe that arcs gracefully towards the tail.

Successful maintenance requires very soft, acidic water — ideally pH below 6.5 and GH below 6 — combined with tannin staining from botanicals, leaf litter, or peat. Under these conditions, the colouration reaches peak intensity. In harder or more alkaline water, colours fade and long-term health suffers.

Feeding on small live and frozen invertebrates is essential: baby brine shrimp nauplii, cyclops, daphnia, and micro-worms produce the best results. High-quality micro-pellets may supplement the diet once the fish is acclimatised. Group sizes of six or more reduce stress and promote natural behaviour.

Water: 23–27°C, pH 4.5–6.5, very soft (GH 1–6, KH 0–3); blackwater botanicals essential. Tank: 40 L minimum; dense planting, floating cover, dark substrate, very gentle flow. Feeding: Baby brine shrimp nauplii, cyclops, daphnia, micro-worms; micro-pellets as supplement. Breeding: Egg-scatterer; soft acidic water critical; condition with live foods; fry tiny. Compatibility: Peaceful nano fish only — no fin-nippers; ideal with dwarf shrimp and small rasboras.