Colorful Pencils for the Freshwater Aquarium

Author: Radek Bednarczuk

The exceptionally charming and vibrant pencilfishes are great schooling fish for the hobbyist looking for something beyond the average tetra. An accomplished fish breeder explains the challenging spawning requirements for Nannostomus mortenthaleri and N. marginatus, an endeavor worth taking on for the experienced aquarist.

A Sea of Color

Imagine a briskly moving school of small, fabulously colorful fish—this is how groups of Nannostomus mortenthaleri and N. marginatus look in our aquarium. Due to their unquestionable charm and beauty, these species can make any tank, even a small one, the center of attention during social gatherings.

The genus Nannostomus (from Greek roots for “small mouth”) belongs to the family Lebiasinidae and is closely related to the characins. It was first described by GÜnther as early as 1872 and at present contains more than a dozen scientifically described species. Many of them exhibit intraspecific differences in coloration due to their extensive distribution: Colombia, Venezuela, Guiana, and French Guiana in the north; the southern Amazon Basin and Bolivia in the south; Peru in the west; and Belem, Brazil in the east.

Species of the genus are similar in appearance in that they all possess black or brown horizontal stripes—except for Nannostomus espei, which has five permanent dark comma-shaped blotches. These fish swim in a horizontal position, although there are two exceptions: N. unifasciatus and N. eques assume an oblique, snout-up posture. With regard to their body shape, the fish in the genus Nannostomusresemble a sharpened pencil 2½ to 5 cm (1 to 2 inches) in length. Despite their small size in aquarium conditions, they can live on average up to five or six years.

Keeping Pencilfish Aquariums

Nannostomus marginatus comes from the waters of South America (the middle reaches of the Amazon), and it can also be found in Venezuela, Peru, Guiana, Surinam, and Colombia. The members of this species are peaceful and sociable fish, although sometimes harmless squabbles can occur between the dominant males. A 50-liter (10-gallon) aquarium at least 60 cm (25 inches) in length can easily keep about 10 fish, as long as it is well planted.

Nannostomus mortenthaleri (known earlier as Nannostomus sp.“Peru” and sometimes also “Peru red”) was caught for the first time not very long ago (in the spring of 2000) in the Rio Nanay, in the vicinity of the town Albarenga in Peru by Martin Mortenthaler, a collector. The fish inhabits the tributaries of the middle and upper reaches of the Rio Nanay in northern Peru, in the Loreto Province.


The males are quite aggressive (territorial), so a larger group of the fish should be kept in order to spread their aggression around. Otherwise the males could kill one another. The aquarium must be densely planted, bigger, and longer than for N. marginatus. Despite their aggressiveness, N. mortenthaleri, especially wild-caught specimens, can be very shy and are best kept with other species of Nannostomus or with other small fish, for instance the common guppy, Endler’s guppy, and/or small characins or cory cats. It should also be remembered that sudden changes in illumination could also make these fish skittish.

Plant Life

In the aquarium these species usually stay at the top or middle levels of the tank. It is advisable to provide floating plants such as Pistia and Riccia, among which the fish feel safe. Another good plant would be the giant Vallisneria, among whose long leaves the fish could find refuge and also lay their eggs. Don’t forget to leave enough room for swimming, though. The bottom of the aquarium should be dark, which will emphasize the beautiful colors of these fish—it is best to cover it with pieces of petrified wood, or use basalt gravel.

Water Characteristics

The physical and chemical parameters to be achieved for these two species are as follows: a temperature of 23° to 26°C (73° to 79°F), pH 6.5 to 7, and the general hardness ideally below 10 degrees. The water is best filtered through peat, and it should also be well aerated. The aquarium should also be equipped with very good biological filtration. Weekly partial water changes should be performed in the amount of 10 to 15 percent. These fish do not like overly large water changes or very strong currents, for instance from a diffuser or spray bar. The aquarium should be covered tightly with a pane of glass, as they are prone to jumping.


The food should be fine-grained, since even for their size these fish have very tiny mouths. As far as live food is concerned, they readily take Artemia, daphnia, fruit flies, gnat larvae, tubifex worms, and small plankton. Dry foods in the form of flakes or granules, which remain at the surface of the water for a long time, are also eaten, but getting wild-caught Nannostomus mortenthaleri to accept these foods will require some patience.

Preparing for Reproduction

In Nannostomus marginatus, sexual dimorphism is well expressed—the males are more slender and more brilliantly colored while the bellies of the females are more rounded. If one plans on breeding this species, for a pair of adult, sexually mature fish it is best to prepare a 10-liter (2½-gallon) tank, 10 cm (4 inches) in height. The aquarium should be sterilized to kill bacteria, as with a weak bleach solution followed by copious rinsing until all chlorine smell is gone. The water should ideally come from a reverse osmosis (RO) filter (it should first be left to stand for about 48 hours). The physical and chemical parameters to achieve for reproduction are pH 5.5 to 6, general hardness of 1 to 3 degrees, and a temperature of 29°C (84°F). It is best to add a few drops of peat extract to the water, which will lower the risk of the eggs fungusing. To achieve the correct hardness, it is best to add ready-made mineral salts in powder form. Non-carbonated mineral water can also be added in correct proportions.


The aquarium should be aerated. It is best to equip it with a small internal filter with a diffuser, filled with a small amount of lava rock (porous filter medium) and seeded with nitrifying bacteria (for biological filtration). If fry are present, another internal filter with a large foam insert should be added to take in particulate impurities. The medium should be cleaned at least twice a day, once in the morning and once in the evening. It should also be remembered that the filter should be turned off while the young are being fed; otherwise it will draw in the food.

Protecting the Eggs

Place a spawning grate, glass marbles, or suitably sized stones on the bottom—these will protect the eggs from being eaten by the adults. On top of these, place a clump of Java moss or an artificial spawning mop in which the eggs will be laid. This substrate should not be too dense, however, as the eggs must be able to fall through to the bottom. The Java moss should be grown in a separate tank, without fish or snails, so that it will be free from pollutants and pathogens. The lower portion of the spawning tank is best covered with a dark, opaque material such as black tape because the eggs are sensitive to light, and the tank itself should be placed in a shady, quiet location that is free from any vibrations.

Choosing Spawning Partners

The problem of choosing a suitable pair for spawning can be easily solved. It is best to buy a whole school of (7 to 10) young fish, preferably from different breeders, and possibly also with some wild-caught, in order to avoid inbreeding. They should be kept in a tank of about 50 to 70 liters (10 to 20 gallons). In a few months you can see the fish spawning among the plants; those pairs should be caught and kept separately or in community tanks and, when the need arises, transferred to the spawning tank to obtain offspring.

Breeding Nannostomus marginatus

When a pair of mature fish is ready to spawn, catch them in the evening and place them in the breeding tank. The spawning, which lasts a few hours on average, should take place on the following day, or the day after at the most. It is advisable to separate the male and female first for about a week and feed them generously with live foods. Their willingness to breed will increase. It should be remembered that the fish should not be fed in the spawning tank itself.

After the spawning, the fish should be caught with a net that has been disinfected. The eggs that remain within the Java moss or spawning mop sometimes fail to fall through, and in such cases it is helpful to shake the spawning media (with a glass rod, for instance) so they fall onto the spawning grate/substrate. From the eggs, which can number up to a hundred, the larvae will hatch in around two to three days—these are so small that they look like glass commas.

After another three days when the yolk sac is absorbed, the smallest kinds of food should be given, including rotifers, or a small amount of egg yolk hard-boiled and mixed with a little yeast. After about three weeks, bigger foods should be provided, such as freshly hatched Artemia, cyclops, and Panagrellus. The fish should be fed a few times a day. It is best to mix the food with a small amount with water, draw it into a hypodermic syringe with a narrow rubber hose attached to it, and then squirt it into the cloud of fry. After feeding, uneaten food should always be removed from the bottom with the syringe, and the tank topped off with water that has the same physical and chemical parameters. Well-fed fish reach around 1 cm (½ inch) at about two months, after which they begin to develop adult coloration.

Breeding Nannostomus mortenthaleri

Nannostomus mortenthaleri does not spawn as frequently as the other species. Distinguishing the sexes depends on the fact that the red rim on the male’s anal fin is darker than that on the female’s. A mature pair should be transferred to a spawning tank, ideally at the time when the male changes color to dark purple, which is a signal that spawning is imminent. As in the previous case, it is best to separate the pairs that have already laid eggs from the rest of the fish, and immediately before spawning isolate the pair from each other (for 7 to 10 days) and feed them generously with quality live food, which will increase their willingness to spawn and, later, the number of fry.

During the spawning itself, the male makes swinging movements, assumes a head-down position, and his color changes from purple to blood red. The pair press against each other, and after a moment of trembling, an egg (sometimes two) falls to the bottom. Usually there are about 30 eggs; the unfertilized ones will disintegrate in time.

Ideal conditions for the reproduction of this species are as follows: general hardness of around 2 degrees, pH 6.8, temperature 24°C (75°F). If you have the opportunity to test the water with a conductivity meter, the conductivity should ideally be around 30 microSiemens. After the spawning is over the fish should be caught, and the procedures as described above for Nannostomus marginatusshould be followed, never forgetting to darken the tank.

At a temperature of 24°C (75°F), after about one and a half to two days, the young should hatch from the eggs, and after another four days they can begin to be fed. The fry of this species are larger than those of Nannostomus marginatus, and they can be given freshly hatched ArtemiaPanagrellus,and mashed algae straight away. While raising the fry of either species, it should be remembered that after about 14 days from their first feeding, the water parameters should begin to be changed gradually, moving towards those in which the adult specimens are kept. After a month, as they keep growing, the young should be moved to a bigger tank. Both species reach sexual maturity at about six to seven months of age.

Benefits of Keeping Pencilfishes

For the imaginative hobbyist, reproduction of these species can bring to mind experiments in a sterile medical lab, and hobbyists should not be discouraged if at first they are only able to raise a few members of the next generation. After some more attempts, one can probably obtain a few dozen or so, which will be a major success and well worth boasting about to other hobbyists. The reproduction of these species would surely be a challenge for any ambitious aquarist.

The fish will reward their keeper for their care with their beautiful colors, which appear at their best against the background of a dark bottom and lush greenery, when the fish are kept in groups numbering from about a dozen to a few dozen specimens. These species are very good for community aquaria and, due to their small size, do not need large tanks. Very often they are merely an addition to planted tanks, but their interesting biology makes it worthwhile to keep them in a species aquarium, or with other members of their genus.

See the full article on TFH Digital