Coral Red Pencilfish, Nannostomus mortenthaleri, Red Arc Pencilfish
Nannostomus mortenthaleri

The brilliant reds of the Coral Red Pencilfish make it an exciting addition to an aquarium!

The Coral Red Pencilfish Nannostomus mortenthaleri is one of the most beautiful species of pencilfish. It is a fairly recent addition to the hobby and was only scientifically described in 2001. It was discovered and named by Martin Mortenthaler, the owner of the Austrian export company, Aquarium Rio Momon, in 2000. This pretty little fish was first described as a sub-species of the Dwarf Pencilfish Nannostomus marginatus but then shortly after was recognized as its own distinct species.

This pencilfish has a red body with two, strong horizontal black lines, giving it a broadly-striped appearance. It is also known as the Red Arc Pencilfish, Peruvian Red Pencilfish, and Ruby Red Pencilfish. This is one of the most beautiful characins available, yet it vies for this position with an even more recently discovered pencilfish relative, the Purple Dwarf Pencilfish Nannostomus rubrocaudatus, which was described in 2009. These two are very similar in color, but only the Coral Red Pencilfish has a white belly.

The males and females are both very colorful, but when the males are displaying to each other, their colors are incredible! Just like Siamese Fighting Fish, the males can be tricked into displaying by the use of a mirror. This pencilfish will believe that its reflection is another male, so it will do its best to show off.

This pencilfish is a very eye-catching, friendly, and active fish. It is relatively small, only reaching barely over 1 inch (2.9 cm), and is a great addition to any community tank containing other small, peaceful fish. It won’t do as well with larger fish, even if they are peaceful, simply because it is easily intimidated and will not be able to compete with them for food. Of course, its small size could also make it a nice little snack for larger angelfish, gouramis, or catfish species.

Coral Red Pencilfish should be kept in a group of at least 6 fish. The males of this species can be aggressive towards each other but usually little damage is done during their fights, especially if the tank has lots of plants. If you plan on having more than one male in your tank, keeping a large group will help discourage aggression. Also, never have more males than females. The males tend to drive the females hard, so keeping a balanced ratio will help prevent any individual females from being stressed. Fortunately, they aren’t aggressive towards other species, so the Coral Red Pencilfish make good community fish.

For Information on keeping freshwater fish, see:
Freshwater Aquarium Guide: Aquarium Setup and Care

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Actinopterygii
  • Order: Characiformes
  • Family: Lebiasinidae
  • Genus: Nannostomus
  • Species: mortenthaleri
Coral Red Pencilfish – Quick Aquarium Care
  • Aquarist Experience Level: Intermediate
  • Size of fish – inches: 1.1 inches (2.90 cm)
  • Minimum Tank Size: 15 gal (57 L)
  • Temperament: Peaceful
  • Aquarium Hardiness: Moderately Difficult
  • Temperature: 72.0 to 82.0° F (22.2 to 27.8&deg C)
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Habitat: Distribution / Background

The Coral Red Pencilfish Nannostomus mortenthaler was described by Paepke and Arendt in 2001. They are found in South America, specifically from the Rio Nanay in Peru, and are also said to be found in the Rio Tigre near Santa Elena. This species is not listed on the IUCN Red List. Other common names they are known by include Red Arc Pencilfish, Peruvian Red Pencilfish, and Ruby Red Pencilfish.

These fish inhabit forest streams, small rivers and their tributaries, as well as swampy areas. The soft acidic waters they live in are shallow and sluggish or still. These habitats usually have dense aquatic vegetation, thick vegetation along the shores, and a substrate of leaf debris and often submerged woody pieces. They are also found in black water areas of seasonally flooded forests and plains.

In the wild the Coral Red Pencilfish are gregarious, yet territorial. They are found in shoals rather than being a school fish. They feed on tiny invertebrates and other zooplankton.

  • Scientific Name: Nannostomus mortenthaleri
  • Social Grouping: Groups
  • IUCN Red List: NE – Not Evaluated or not listed


The Coral Red Pencilfish, true to its “pencilfish” name, has an elongated body that is pointed at the snout. It will grow to just over 1 inch (2.9 cm) and has an expected lifespan of 5 years. Its beautiful red body features two strong, horizontal black lines, giving it a broadly-striped appearance. The lower portion is paler and often white. Males are more brightly-colored than females and have a white patch at the base of the dorsal fin.

  • Size of fish – inches: 1.1 inches (2.90 cm)
  • Lifespan: 5 years

Fish Keeping Difficulty

The Red Arc Pencilfish is a bit difficult to keep and recommended for aquarists with fish keeping experience. Most are wild-caught specimens, which tend to be less tolerant of varying water conditions than commercially-bred fish. These freshwater pencilfish are prone to ich, especially when first introduced into a new aquarium. They should be acclimated in a quarantine aquarium before being placed in their permanent home.

  • Aquarium Hardiness: Moderately Difficult
  • Aquarist Experience Level: Intermediate

Foods and Feeding

The Red Arc Pencilfish are omnivorous. Though in the wild they feed primarily on tiny invertebrates and other zooplankton, in the aquarium they will generally eat all kinds of foods. This fish is not a picky eater, so it will eat most prepared fish foods, including small pellets, freeze dried foods, and most flakes foods. To keep them looking their best, brine shrimp (live or frozen) or frozen blood worms should be fed occasionally. Keep in mind that their mouths are very small, so foods need to be crushed or minced in order for them to be able to eat it.

  • Diet Type: Omnivore – Their mouths are very small, so foods need to be crushed or minced.
  • Flake Food: Yes
  • Tablet / Pellet: Yes
  • Live foods (fishes, shrimps, worms): Some of Diet
  • Vegetable Food: Some of Diet
  • Meaty Food: Some of Diet
  • Feeding Frequency: Several feedings per day

Aquarium Care

Coral Red Pencilfish are moderately hardy and not exceptionally difficult to care for provided their water is kept clean. Water should be changed on a regular basis, especially if the tank is densely stocked. At least 25 to 50% of the tank water should be replaced every other week.

  • Water Changes: Bi-weekly

Aquarium Setup

The Coral Red Pencilfish is more difficult to keep than many other tetras. The water conditions should be kept soft and acidic for them to thrive. Peat filtration is advisable as this blackwater native is very intolerant of harder and more alkaline water. These fish are best kept in groups or 6 or more, so they need about a 15 gallon long tank or a 20 gallon aquarium. They will swim and feed mostly at the surface of the water. The tank should be tightly sealed as this fish is apt to jump if provided the opportunity.

The aquarium should be dim with a dark substrate to bring out this fish’s best colors. The aquarium should also be well-planted with some floating plants at the surface to help subdue the light. Although they will appreciate some floating plants, because they eat at the surface, some clear areas need to be provided as well. Driftwood decor will help provide these fish plenty of places for retreat.

  • Minimum Tank Size: 15 gal (57 L) – They need at least a 15 gallon long or 20 gallon aquarium.
  • Suitable for Nano Tank: Yes
  • Substrate Type: Any
  • Lighting Needs: Low – subdued lighting
  • Temperature: 72.0 to 82.0° F (22.2 to 27.8&deg C)
  • Breeding Temperature: 75.0° F – These fish will spawn at about 75° F (2° C).
  • Range ph: 5.8-7.0
  • Hardness Range: 2 – 10 dGH
  • Brackish: No
  • Water Movement: Moderate
  • Water Region: All – The Coral Red Pencilfish will swim in most parts of the aquarium.

Social Behaviors

The Coral Red Pencilfish can be kept singly, but like many characins, they are a shoaling fish and more comfortable with a group of their own kind. They will be best kept in a group of at least 6 fish. Males will rival with each other, but a heavily planted aquarium will help prevent any serious damage. If you plan on having more than 1 male in your tank, you should keep as many as you can to keep aggression to a minimum. There should also be fewer males than females, or an equal number of each, as males tend to drive the females hard. Too much constant pestering can stress individual females.

Fortunately, these fish aren’t aggressive towards other species, so the Red Arc Pencilfish make a good community fish. However, they will be better with other smaller fish. They can be intimidated by larger, peaceful fish and won’t be able to compete with them for food. Of course, their small size could also make them a nice little snack for larger fish like angelfish, gouramis, or catfish species.

  • Venomous: No
  • Temperament: Peaceful
  • Compatible with:
    • Same species – conspecifics: Yes – These fish are best kept singly or in groups of 6 or more. Males can be aggressive towards each other, so schools should be as large as possible to spread out aggression.
    • Peaceful fish (): Safe – They are best with other smaller, peaceful fish as they are easily intimidated.
    • Semi-Aggressive (): Threat
    • Aggressive (): Threat
    • Large Semi-Aggressive (): Threat
    • Large Aggressive, Predatory (): Threat
    • Monitor – Characins can out compete them for food.
    • Shrimps, Crabs, Snails: Safe – not aggressive
    • Plants: Safe

Sex: Sexual differences

The males have an obvious white patch at the base of their dorsal fin. To sex these fish, look at the outside edge of the anal fin, which is a bright, deep red in males and much paler in females. During spawning, the male’s overall color will become much brighter. Another method of sexing is to watch their behavior. Males will frequently display and nip at each other while the females are very docile.

Breeding / Reproduction

The Coral Red Pencilfish doesn’t spawn as frequently as other species, but it has been bred in captivity. Although it has not been bred on a commercial basis, aquarists have had successful spawnings. The breeding process is difficult in the home aquarium, but not impossible with the right conditions. Adults in good condition have been known to spawn in a mature, densely planted community tank, but unfortunately the eggs and fry are most often eaten.

A separate breeding tank with soft, acidic water is best for a successful spawn and to get the largest number of fry. The adults need to be in good condition. Males get aggressive with one another when breeding and defending a territory, so it may be best to spawn them in pairs. More than one pair can be bred together, however, if the tank is large enough and offers clumps of spawning media arranged to create individual territories. When in a spawning mood, the males usually just display at each other, devoting most of their time to looking for females. If they do get too aggressive, though, you will want to separate them.

A smaller, 3 to 5 gallon breeding tank is adequate for a single pair, but a larger tank, up to 10 ten gallons, may be necessary for multiple pairs. The breeding tank does not need light as these fish are sensitive to changes in light. However, the tank should be heavily planted. Provide many fine-leaved plants, such as java moss, or clumps of fine wool or nylon spawning mops where the females can deposit the eggs. The water should be soft and slightly acidic with a pH of 6.8, a hardness of about 2°, and a temperature of about 75° F (24° C). Filtration is not essential, but a small, air-powered sponge filter can be added if desired.

Condition adult breeders for 7 to 10 days by offering plenty of small, live foods. They can be conditioned as a group, or choose 1 to 3 male/female pairs and isolate each male and female separately. Conditioning them separately will increase their willingness to spawn and also stimulate a greater number of eggs. Males will change to a dark purplish color when they are willing to spawn, and that is the ideal time to transfer them into the breeding tank.

For the next few days, the fish will spawn each day. If no spawning occurs after about 3 days, move them back into their conditioning tank for a few more days and try again. When spawning, the male will make swinging motions with his body and assume a head down position while his color changes from purple to deep red. The pair will press together, and amidst quivering motions, an egg or two will be released and fall into the spawning media. The female will usually lay up to about 30 eggs.

Once a successful spawn has been achieved, remove the parents and keep the tank darkened. The eggs hatch in approximately 36 hours, and the fry become free-swimming 4 to 5 days later. They can feed on micro worm or brine shrimp nautili. They will develop the adult patterning after about 8 weeks and reach sexual maturity in about 6 to 7 months. See Breeding Freshwater Fish: Characins for a general description of breeding processes, and see Fish Food for Fry for information about types of foods for raising the young.

  • Ease of Breeding: Difficult

Fish Diseases

As with most fish, Coral Red Pencilfish are prone to skin flukes, parasitic infestations (protozoa, worms, etc.), parasitic infestations (protozoa, worms, etc.), bacterial infections (general), and bacterial disease. Anything added to a tank can introduce disease. Not only other fish, but plants, substrate, and decorations can harbor bacteria. Take great care and make sure to properly clean or quarantine anything that you add to an established tank so as not to upset the balance.

A good thing about the Coral Red Pencilfish is that due to their resilience, an outbreak of disease can often be limited to just one or a few fishes if dealt with at an early stage. Stressed fish are more likely to acquire disease. The best way to proactively prevent disease is to provide the proper environment and a well-balanced diet.

The more closely their environment resembles their natural habitat, the less stress the fish will have, making them healthier and happier. It is recommended to read up on the common tank diseases. Knowing the signs of common tank diseases and treating them early makes a huge difference.For information about freshwater fish diseases and illnesses, see Aquarium Fish Diseases and Treatments.


These characins are sold as Coral Red Pencilfish as well as under the name of Red Arc Pencilfish, Peruvian Red Pencilfish, and Ruby Red Pencilfish. This is a relatively rare species, and the price can be quite high. The price may drop, however, now that it has been tank-spawned and may eventually be commercially-bred.


 Red Pencilfish (Image Credit: Brian Gratwicke, Wikimedia Commons CC BY-SA 2.0 Generic)