Red Arc Pencilfish

Nannostomus mortenthaleri requires very small foods due to its tiny mouth; staple diet should consist of micro live or frozen foods such as baby brine shrimp, micro worms, Daphnia, and Moina. High-quality micro-pellets and crushed flake can supplement but should not form the primary diet. Variety is essential for maintaining vibrant coloration and long-term health.

This species is a shoaling fish that displays its best coloration and most natural behavior when kept in groups of eight or more individuals. Males establish loose dominance hierarchies through lateral displays and fin-flaring, which is visually impressive and generally harmless. The species is timid around boisterous or fast-moving tankmates and will become stressed and reclusive if not kept in a calm, well-planted environment.

Breeding requires very soft, acidic blackwater conditions with pH ideally between 4.5 and 5.5 and near-zero hardness, often achieved with RO water conditioned with botanicals or peat. Eggs are scattered among fine-leaved plants such as Java moss or spawning mops and receive no parental care, so adults should be removed or the eggs transferred to a separate rearing tank to prevent predation. Fry are extremely small and require infusoria or commercial fry foods for the first week before graduating to micro worms and baby brine shrimp.