Care guide · Freshwater · fish

Coral red pencilfish

Nannostomus mortenthaleri

Coral red pencilfish, photo Brian Gratwicke, CC BY 2.0. Photo: Brian Gratwicke · Source · CC BY 2.0Resized and compressed with full original framing and aspect ratio preserved.

The coral red pencilfish (Nannostomus mortenthaleri) is a small Peruvian species whose adult males develop vivid red colour. Its size is misleading: males can seriously injure one another in cramped aquaria. A mixed-sex group needs at least a 90 × 30 cm footprint, broken sight lines and a shoal of around ten or more. The fish is shy, needs stable soft water and eats small prey. FishBase reports an IUCN status of Critically Endangered; seek documented captive-bred stock only and do not encourage wild collection.

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At a glance

Temperature: 24–28 °C
pH: 4.0–7.0
GH: 1–5 °dGH
Aquarium length: At least 90 cm
Minimum aquarium volume: At least 81 L
Adult size: At least 3 cm
Group size: At least 10 individuals
Temperament: Peaceful

About this species

The accepted name is Nannostomus mortenthaleri Paepke & Arendt, 2001, in Lebiasinidae. It was first described as N. marginatus mortenthaleri; later research raised it to full species rank. That historical combination is not a second species.

Adults reach about 30 mm standard length. Another aquarium source gives 3.9 cm total length; these differ by convention because total length includes the caudal fin. Males are slimmer and more colourful than females, with coral-red pigment mainly between the upper two dark lateral stripes.

Natural habitat

The species is known from Loreto, Peru, with its type locality in a small tributary of the Rio Nanay. Specialist accounts also describe sites in the Rio Tigre system. Its known distribution is restricted; do not infer a broader range than the sources support.

It inhabits quiet shallow streams, swampy areas and flooded forest among marginal plants, branches and leaf litter. Rio Tigre field sites measured pH 4.54–5.78 and 25.3–26.8 °C. These are measurements from particular places, not values to reproduce suddenly in an aquarium.

Aquarium and layout

For a mixed-sex group, Seriously Fish specifies at least a 90 × 30 cm footprint and about 81 litres. That is larger than its small body might suggest because rival males can seriously injure one another in confined quarters. A group of ten or more is recommended, but the aquarium must still be well structured.

Plant the margins densely, add wood and optionally floating plants, and create several broken sight lines so subordinate fish can retreat. Keep swimming routes open. Use gentle filtration, cover the tank and never add the fish to an immature aquarium. A species-focused setup may suit it better than a generic community tank.

Water conditions

Seriously Fish lists 24–28 °C, pH 4.0–7.0 and GH 1–5 dGH. Rio Tigre field readings of 25.3–26.8 °C and pH 4.54–5.78 fall within those ranges. The sources support soft, acidic water; stable conditions remain the priority. FishBase gives no separate aquarium range.

Do not lower pH or hardness rapidly with chemicals. Match acclimation to the water used for captive-bred fish and avoid fluctuations. No reliable species-specific KH target was found.

Behaviour and group size

The fish is peaceful toward other species, but adult males are territorial and may spar during the day. It is therefore not an ideal general community fish. Adequate floor space, ten or more fish and broken sight lines are important; in a small aquarium injuries or deaths can occur.

Prefer a species-focused setup or very small, calm companions that do not compete for food. Large, fast or boisterous fish can suppress shy individuals. Shared water values alone do not prove compatibility. Watch fins, appetite and hiding, and separate fish if aggression persists.

Feeding

In nature N. mortenthaleri is a micropredator of tiny invertebrates and zooplankton. Offer small live or frozen prey daily, such as Artemia nauplii, Moina and grindal worms. Suitable fine dry food may be accepted, especially by captive-bred fish, but confirm that every individual eats it.

The small mouth needs a suitable particle size; a large flake or pellet is not automatically appropriate. Distribute small portions calmly in more than one place and remove leftovers. No product is linked until the existing review confirms suitable size and purpose.

Breeding

Aquarium breeding has been reported but should not be presented as routine or guaranteed. A separate mature tank with fine spawning material, such as moss or a spawning mop, helps keep eggs away from adults. The species scatters eggs; adults may eat them. Remove the adults or transfer the spawning material when eggs are found.

First foods must be extremely small, such as Paramecium or suitable fine fry food. Add Artemia nauplii and microworms only when fry can take them. Published timings are source-specific, not promises. Use captive-bred fish and keep water and lighting stable.

Sources & review

This care guide draws on several specialist sources and was last checked on 16 October 2026.

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