Nannostomus mortenthaleri

Common Names: Red arc pencilfish, Peruvian red pencilfish, coral red pencilfish, ruby red pencilfish

Type Locality: Peru: a small tributary of Rio Nanay at the village of Alvarenga, Loreto, Province of Maynas (Iquitos)

Range: Apparently very limited in range

Taxonomic Troubles: Originally described as Nannostomus marginatus mortenthaleri

Size: 3.9 cm TL (1.5 inches)

Preferred Water Chemistry: Tropical, soft, slightly acidic.

Difficulty: A hardy pencil, quite assertive for its size.

Tank Setup: This is an active, schooling species that should always be kept in groups, which will also diffuse aggression. A planted tank is best. Can be kept with any other fish that are not big enough to eat them.

Feeding: A micropredator. Relishes live or frozen foods, tank-bred specimens will readily take all regular aquarium fare.

Breeding: This species is only starting to be bred by aquarists. It does not appear to be too difficult to spawn.

Fish Description

Widely heralded as the all-red pencilfish, this species looks like N. marginatus painted red. Males are more colorful, but females are not drab.


Known as the "$20 pencilfish," this is an expensive fish but still in demand because of its outstanding coloration. As captive-bred fish are becoming available, the price is dropping. The intense red coloration of this species has caught the attention of aquarists who are not generally interested in the lebiasinids—splashing tetras and pencilfish. The soaring popularity of planted nano tanks has also added to the demand for this fish, which is perfect for such a setup. It is reportedly extremely limited in range and population, but fortunately the availability of tank-bred specimens has lessened the demand for wild harvesting.