Furry Diary

Not furry, but we like them anyway.

Coral Red Pencilfish

Nannostomus mortenthaleri

Also called Red arc pencilfish, Peruvian red pencilfish, Ruby red pencilfish, Red pencilfish, Purpurziersalmler

SpeciesShort profile

From Peru: the Rio Nanay basin and Rio Tigre system in Loreto, near Iquitos12

At a glance the quick version

Length2.9 to 3 cm12
Sizetiny
Striking ruby-red colour in males
Peaceful toward other species
Lively displays between males
1

Looks how to spot one

Short, stocky scaled characin

A small, compact pencilfish with three bold dark stripes. Males glow a vivid red from the snout to the tail, with a white mark and red tip at the front of the dorsal fin. Females are plumper and lack the red body colour. At night both sexes turn much paler while the stripes stay visible.13

Coloursbright redblack stripeswhite dorsal-fin marking1

Personality what they are like

Peaceful toward other kinds of fish but shy because of its small size, so it suits a quiet species-focused tank or a few calm companions such as Apistogramma. Males are more spirited than most pencilfish and hold small territories, sparring with each other.13

Daily activity how busy a day looks

Active among plants in the middle and lower water, with males patrolling and displaying.1

Grooming and care the hands-on bits

Needs very soft, acidic water. Suggested ranges: 24-28 C, pH 4.0-7.0, hardness 1-5 dGH. Keep water clean and stable with gentle filtration.1

Home where they are happiest

Tank of at least 90 cm length (about 81 litres) with dense planting, wood and leaf litter that break up sightlines so each male has his own space.13

Food basics what goes in the bowl

A micropredator of tiny invertebrates and zooplankton. Offer small dried foods along with live or frozen brine shrimp nauplii, moina and grindal worms.1

Company who they like around

Best in a group of 10 or more, with plenty of cover so the males can share space comfortably.1

The story where it all began

First described in 2001 as a subspecies of the dwarf pencilfish, it was raised to full species status in 2003. It is named after Martin Mortenthaler, an Austrian fish exporter who collected the first specimens.12

Fun facts for the dinner table

  1. 1

    It was named after Martin Mortenthaler, the exporter who discovered it.2

  2. 2

    It is known from only one small part of Peru, near the city of Iquitos.1

  3. 3

    It was first treated as a subspecies of the dwarf pencilfish before being recognised as its own species in 2003.1

  4. 4

    It belongs to the stubby-bodied pencilfish group rather than the long, slim ones.3

Recognised by the official bits

IUCN Red List
Critically Endangered (assessed April 2014)2
FishBase
Described by Paepke & Arendt in 20012

The care rhythm Furry Diary remembers it for you

A typical home-care rhythm for a Coral Red Pencilfish. It is our own starting plan, not a vet's advice, and you can change every line in the app.

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Where this comes from

Every breed page shows where each fact comes from. If we got something wrong, sorry, we are pet people, not a science magazine. Tell us at [email protected] and we will fix it.

  1. Nannostomus mortenthaleri - Seriously Fish seriouslyfish.comused for origin, size, care, diet, behaviour, history
  2. Nannostomus mortenthaleri summary - FishBase fishbase.seused for size, IUCN, etymology, names
  3. Nannostomus - Wikipedia en.wikipedia.orgused for body shape, genus lifespan

Checked 2026-09-30. Text is written in our own words from these sources.