Coral Red Pencilfish Care Guide: Tank Setup, Diet, Tank Mates, and More

Table of Contents

• Key Takeaways
• Species Overview
• Classification
• Origin & Natural Habitat
• Appearance & Identification
• Average Size & Lifespan
• Care Guide
• Tank Mates
• Food & Diet
• Breeding & Reproduction
• Common Health Issues
• Common Mistakes to Avoid
• Where to Buy
• Frequently Asked Questions
• Closing Thoughts
• Recommended Video
• References

The coral red pencilfish is, in my opinion, one of the most beautiful small fish in the freshwater hobby. It has intense red and black banding that looks almost painted on, and in the right conditions, the red becomes so vivid that it’s hard to believe you’re looking at a real fish. This species was only described scientifically in 2001, making it a relatively recent addition to the aquarium world.

Found only in a handful of small streams in Peru, the coral red pencilfish is a specialty species that rewards dedicated keepers with stunning coloration and fascinating territorial behavior. It’s not the easiest pencilfish to keep, but for anyone willing to meet its needs, it’s absolutely worth the effort. Here’s everything you need to know.

Key Takeaways

• Arguably the most colorful pencilfish species with striking red and black bands
• Males are territorial and display constantly, making them fascinating to watch
• Soft, acidic water is essential for health and coloration
• Small species (1.2 inches max) suitable for nano tanks of 10+ gallons
• Micropredator that prefers live and frozen foods over dry food

Species Overview

Field	Details
Scientific Name	Nannostomus mortenthaleri
Common Names	Coral Red Pencilfish, Red Arc Pencilfish, Mortenthal’s Pencilfish
Family	Lebiasinidae
Origin	Rio Nanay basin, Loreto, Peru
Care Level	Moderate
Temperament	Peaceful but territorial among males
Diet	Omnivore (micropredator)
Tank Level	Mid to Top
Maximum Size	1.2 inches (3 cm)
Minimum Tank Size	10 gallons (38 liters)
Temperature	75-82°F (24-28°C)
pH	4.0-7.0
Hardness	0-8 dGH
Lifespan	3-5 years in captivity
Breeding	Egg scatterer
Breeding Difficulty	Moderate to Difficult
Compatibility	Specialist community
OK for Planted Tanks?	Yes

Classification

Taxonomic Level	Classification
Order	Characiformes
Family	Lebiasinidae
Subfamily	Pyrrhulininae
Genus	Nannostomus
Species	N. mortenthaleri (Paepke & Arendt, 2001)

This species was described by Hans-Joachim Paepke and Klaus Arendt in 2001, making it one of the most recently described pencilfish species. It was named after Martin Mortenthaler, a Peruvian aquarium fish exporter who first collected and brought the species to scientific attention.

Note on family placement: Pencilfish belong to Lebiasinidae, which is separate from Characidae. This family was not affected by the 2024 Melo et al. revision. Lebiasinidae has been taxonomically stable, and Nannostomus is universally recognized as the pencilfish genus.

Origin & Natural Habitat

The coral red pencilfish has an extremely limited natural range. It’s found only in the Rio Nanay drainage near Iquitos in Loreto, Peru, within the upper Amazon basin. This restricted distribution makes it one of the more geographically limited aquarium species.

In the wild, they inhabit small, slow-moving forest streams and pools with dense aquatic and riparian vegetation. The water is typically very soft, acidic, and stained with tannins from decomposing plant matter. The substrate is covered in leaf litter, and fallen branches provide additional structure and territories for the fish.

The limited range of this species means wild populations could be vulnerable to habitat destruction. Most fish in the aquarium trade are now captive-bred, which is both good for wild populations and beneficial for aquarists since captive-bred fish adapt more readily to aquarium conditions.

Appearance & Identification

The coral red pencilfish is widely regarded as the most colorful member of the Nannostomus genus. Males display three bold horizontal bands: broad red stripes alternating with black stripes, running the length of the body. The red is an intense coral to crimson that deepens with the fish’s mood and condition. When males are displaying to each other, the coloration reaches its peak intensity.

The body shape is typical of pencilfish: elongated, cylindrical, and streamlined. Like other Nannostomus species, they often hover at an oblique angle in the water, which is a characteristic pencilfish behavior rather than a sign of illness.

Sexual dimorphism is easy to see in this species. Males are brilliantly colored with the full red and black pattern, while females are considerably more subdued. Females show a brownish-olive body with faint striping and lack the intense red coloring. Males also develop slightly more elongated body proportions.

Like all pencilfish, N. mortenthaleri displays a nocturnal color pattern that appears when the lights go off. The bold stripes fade and are replaced by a pattern of vertical bars. This is normal and not a cause for concern.

Average Size & Lifespan

This is a small species, reaching a maximum of about 1.2 inches (3 cm). Their small size makes them suitable for nano and small community tanks, though their water chemistry requirements add complexity to the setup.

With proper care in appropriate water conditions, expect a lifespan of 3 to 5 years. Soft, acidic water, a varied diet, and a well-structured tank are the keys to longevity.

Care Guide

Tank Size

A 10-gallon tank is the minimum for a small group. For a group of 8 to 12 with other small tank mates, a 20-gallon long is ideal. These fish don’t need a lot of swimming space, but territorial males need enough room to establish their own small domains without constant conflict.

Water Parameters

Parameter	Ideal Range
Temperature	75-82°F (24-28°C)
pH	4.0-7.0 (prefers acidic)
General Hardness	0-8 dGH
KH	0-4 dKH
Ammonia / Nitrite	0 ppm
Nitrate	Below 15 ppm

Soft, acidic water brings out the best coloration and supports long-term health. RO/DI water remineralized to target parameters is the most reliable approach. Indian almond leaves, alder cones, and driftwood help naturally acidify the water and release beneficial tannins. The darker and softer the water, the more vivid the reds become.

Water quality must be excellent. These fish are not tolerant of elevated nitrates or organic waste. Small, frequent water changes (15 to 20 percent twice weekly) work better than large weekly changes for maintaining stability.

Tank Setup

A well-planted tank with lots of visual barriers is ideal. Use driftwood branches, dense plants, and leaf litter to create separate micro-territories for males. Without adequate structure, dominant males will relentlessly harass subordinates.

Keep the lighting subdued. Floating plants are highly recommended to diffuse light and add security. Low-light plants like Java fern, Anubias, Cryptocoryne, and mosses work well in these conditions.

A dark substrate brings out the best coloring. Black sand is a popular choice that provides excellent contrast with the fish’s red and black pattern.

Tank Mates

Coral red pencilfish can be kept in a specialist community with carefully selected tank mates. They’re peaceful toward other species but males are territorial toward each other.

Good Tank Mates

• Small, peaceful tetras (ember tetras, green neons, cardinal tetras)
• Other Nannostomus species (though avoid similar-looking species to prevent hybridization)
• Pygmy corydoras, habrosus corydoras
• Otocinclus
• Dwarf Apistogramma species
• Small rasboras
• Cherry shrimp (adults are generally safe)

Tank Mates to Avoid

• Active, boisterous species that would stress them or outcompete for food
• Large or aggressive fish
• Species requiring hard, alkaline water

Food & Diet

Coral red pencilfish are micropredators in the wild, feeding on tiny invertebrates, insect larvae, and zooplankton. They have small mouths and prefer small food items.

• Best foods: Live baby brine shrimp, daphnia, grindal worms, microworms
• Frozen foods: Cyclops, baby brine shrimp, daphnia
• Dry foods: High-quality micro pellets or crushed flakes (some individuals accept these, many prefer live/frozen)

Feed small amounts two to three times daily. A diet heavy in live foods brings out the most intense coloration, especially in males. If you want to see these fish at their absolute best, a regular supply of live baby brine shrimp is the way to go.

Breeding & Reproduction

Coral red pencilfish can be bred in captivity, though it requires some effort and attention to water conditions.

Breeding Setup

• Breeding tank: 5 to 10 gallons with very dim lighting
• Water: Very soft, acidic (pH 5.0 to 6.0, near-zero hardness)
• Temperature: 79 to 82°F (26 to 28°C)
• Decor: Fine-leaved plants like Java moss, or spawning mops
• Filtration: Gentle sponge filter

Males court females with intense color displays and body posturing. Eggs are deposited among fine-leaved plants, typically a few at a time over the course of several days. Adults will eat eggs if they find them, so dense plant cover is essential to protect some of the spawn. Eggs hatch in approximately 24 to 36 hours, and fry are extremely tiny, requiring infusoria or paramecium as first food.

Common Health Issues

• Water chemistry stress: Keeping them in hard, alkaline water leads to washed-out coloring, stress, and increased disease susceptibility.
• Ich and velvet: Can occur during acclimation or after temperature swings.
• Starvation: Their small mouths mean they can’t eat large food items. Make sure food is appropriately sized.
• Male aggression: Dominant males can harass subordinates in tanks without adequate structure. Provide plenty of visual breaks.

Common Mistakes to Avoid

• Hard, neutral water: The most common mistake. These fish need soft, acidic conditions to show their true colors and stay healthy.
• Food too large: They have tiny mouths. Standard-sized bloodworms and pellets may be too big. Use micro-sized foods.
• Not enough cover: Males need visual barriers to establish territories. Without them, one male will dominate and stress the rest.
• Keeping only males: While the males are more colorful, an all-male group leads to constant aggression. Include females to diffuse tension.
• Bright lighting: Washes out their coloration and makes them shy. Subdued lighting is essential.

Where to Buy

Coral red pencilfish are a specialty species available from dedicated online retailers and importers. They’re not common at chain pet stores but can be found through these trusted sources:

• Flip Aquatics
• Dan’s Fish

Frequently Asked Questions

Why do my coral red pencilfish look dull?

The most common reason is water that’s too hard or alkaline. These fish need soft, acidic water to display their best coloration. Other factors include bright lighting, stress from aggressive tank mates, and a diet lacking in live foods. Fix the water chemistry first, and the color should improve dramatically.

Are coral red pencilfish aggressive?

Males are territorial toward each other, but they’re peaceful toward other species. The territorial displays between males are actually one of the most entertaining aspects of keeping this species. Provide enough structure for each male to claim a small territory, and the aggression stays at the level of displaying and posturing rather than real damage.

Can I keep coral red pencilfish in a nano tank?

Yes. A 10-gallon tank is suitable for a small group of 6 to 8 fish. The main challenge isn’t space but rather maintaining stable, soft, acidic water chemistry in a small volume. Larger tanks are actually easier to manage from a water quality perspective.

Closing Thoughts

The coral red pencilfish is a tiny fish that delivers an outsized visual impact. A group of males displaying to each other in a dimly lit blackwater tank, with their reds blazing against a dark background, is one of the most beautiful sights in freshwater fishkeeping. It’s the kind of fish that makes you lean in and stare.

The care requirements are specific but not complicated. Soft water, acidic pH, small foods, dim lighting, and plenty of plant cover. If you’re already running a blackwater setup or willing to invest in RO filtration, adding coral red pencilfish is one of the best decisions you can make. They’re small, but they’re absolutely unforgettable.

Recommended Video

Check out our Tetra Tier List video where we rank popular tetra species for the home aquarium:

References

• Froese, R. and D. Pauly, Eds. FishBase. Nannostomus mortenthaleri. Accessed 2025.
• SeriouslyFish. Nannostomus mortenthaleri species profile. Accessed 2025.
• Paepke, H.-J. and K. Arendt (2001). Description of a new species of the genus Nannostomus from Peru. Zoologische Abhandlungen, 51(3), 33-38.

Explore More Tetras

The coral red pencilfish is just one of dozens of characin species we cover in our complete species directory. Whether you’re into vibrant nano fish or classic community species, our guide has you covered.

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