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Dwarf Pencilfish

Scientific Name: Nannostomus marginatus
Family: Lebiasinidae

About the Dwarf Pencilfish

Species Type: Freshwater Fish
Category: Characins

Care Level: Moderate. May tolerate only a narrow range of water parameters, have specific dietary requirements including frozen or even live foods, may have behaviors that severely limit potential tankmates or may require a specialized aquarium setup.

Origin: Widespread over the Amazon basin in tributary streams of the Rio Negro; also in Guyana and Suriname. Occurs in slow-flow streams and swampy pond areas.

Compatibility/Temperament: Peaceful tetra-like fish, suitable in a species tank or in a community tank of similar small quiet characins, rasbora, small catfish and loaches; good with dwarf cichlids as it tends to remain in the upper half of the tank. Should not be kept with active fish that will be seen as threatening.


A lovely colourful miniature fish worthy of its own species biotope tank or in a quiet well-planted community aquarium. Must be kept in a group, at least half a dozen but preferably more, with a good ratio of females to males to avoid males driving the females too hard. Males will challenge each other with no damage. The species prefers the middle and upper levels in the aquarium, swimming and browsing plant leaves.

Females are stockier than males. Spawning will occur in very soft acidic water in a well-planted tank, and follows the standard characin method. Adults will eat the eggs if not removed.

This species is a widespread polymorphic one, with distinct populations occurring in the Guyanas and in several widely-separated areas in the Amazon basin (Weitzman, 1966). At least three distinct colour variants are known. When first discovered, the beautiful coral red dwarf pencilfish [now a distinct species, Nannostomus mortenthaleri] was originally described as a subspecies [see that profile]. The original description by C.H. Eigenmann in 1909 used specimens collected in Maduni Creek, Guyana. The populations from the lower and upper Amazon basins may prove to be sufficiently different after further study to merit subspecific recognition (Weitzman, 1966).

Nannostomus marginatus does not possess an adipose fin. In common with all pencilfish, the fish has a small terminal mouth that is always open, and (except for N. espei) a diurnal colour pattern. During darkness, the horizontal lines break up into a series of dashes. This has been observed in blind fish, showing that it is an automatic response and not controlled by the fish.

All pencilfish are found in the tribe Nannostomini in the subfamily Pyrrhulininae. Three different genera (Nannostomus, Nannobrycon and Poecilobrycon) were used at various times until Weitzman & Cobb (1975) placed all species in the single genus, Nannostomus, erected by Gunther in 1872 with N. beckfordi as the type species; Gery (1977) separated them into two genera, Nannostomus and Nannobrycon, largely on the basis of the swimming position. Weitzman & Weitzman in Reis et al. (2003) reassigned the species into the single genus Nannostomus which now includes all described pencilfish. The genus name Nannostomus comes from the Greek meaning "small mouth."


Gery, Jacques (1977) "Characoids of the World," TFH Books.

Reis, R.E., S.O. Kullander & C.J. Ferraris Jr. (2003), "Check list of the freshwater fishes of South and Central America."

Weitzman, Stanley H. (1966), "Review of South American Characid Fishes of Subtribe Nannostomina," Proceedings of the United States National Museum (Smithsonian Institution), Volume 119, Number 3538.

Weitzman, Stanley H. & J.S. Cobb (1975), "A revision of the South American fishes of the genus Nannostomus Gunther (family Lebiasinidae)," Smithsonian Contributions to Zoology No. 186.

Dwarf Pencilfish Diet

Usually will accept most prepared foods like flake and frozen daphnia and bloodworms. Live food such as wingless fruit flies would be a treat. The mouth is very small.


Maximum 1.4 inches.

Minimum Tank Suggestion

10 gallons

Ideal water parameters for Dwarf Pencilfish

Soft (hardness less than 10 dGH) acidic (pH below 7.0) water, temperature 24-26C/75-79F. Wild-caught fish will not last in basic or harder water, and tank-raised fish may survive but will not be as colourful.

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