Coral red pencilfish
Nannostomus mortenthaleri
Also known as Red Arc Pencilfish
FreshwaterFish
Care at a glance
- Temperature
- 75–82°F
- 24–28°C
- pH
- 5.5–7.0
- Adult size
- 1.2 in
- 3 cm
- Minimum tank
- 15 gal
- 55 L
- Temperament
- Peaceful
- Social behavior
- Shoaling
- Swim level
- Middle
- Difficulty
- Medium
- Breeding
- Medium
- Rarity
- Rare
- Family
- Lebiasinidae
- Origin
- Peru
Overview
Keep them in a shoal of six or more in a small planted tank of about 55 liters, where the males will color up and spar over rank without doing harm. They hold the middle of the water and are peaceful, so pair them only with other gentle, small species. Aim for the mid to high 20s C and a soft, acidic to neutral pH. Their mouths are small, so feed micro foods like crushed flake, micro-pellet, and tiny live or frozen items.
In the wild
Coral red pencilfish come from slow, clear streams in the Peruvian Amazon, around the Rio Nanay. The water is soft and acidic, full of leaf litter, roots, and fine plants. A blackwater-style setup with dim light, tannins, and lots of planting suits them and brings out their red. Gentle flow only, since these are small, delicate fish.
Good to know
This pencilfish sits in the family Lebiasinidae and stays tiny at around 3 cm. Males show the deepest red, while females are paler and rounder. It was only described in 2001, and it is a micropredator that eats very small invertebrates in the wild. Breeding is possible but takes effort.
Did you know
Rival males put on a near-constant color show, sparring and flaring to sort out rank, and they turn their brightest red during these standoffs. A single male kept alone never reaches that intensity because there is nobody to compete with.
Weekly picks
Tank mates
Coral red pencilfish is a peaceful, shoaling species that keeps to the middle. These picks share a community profile with it, are safe together in both directions, and overlap on water type, temperature and pH.

Green lace shrimp
Atyoida pilipes
Freshwater2.8 in (7 cm) · 14 gal (54 L)73–82°F (23–28°C)
Martin's dwarf cichlid
Apistogramma martini
Freshwater1.6 in (4 cm) · 15 gal (55 L)73–81°F (23–27°C)
New Guinea blue-eye
Pseudomugil novaeguineae
Freshwater1.8 in (4.5 cm) · 15 gal (55 L)79–86°F (26–30°C)
Harlequin rasbora
Trigonostigma heteromorpha
Freshwater2 in (5 cm) · 15 gal (55 L)72–82°F (22–28°C)
Volcano cory
Osteogaster hephaestus
Freshwater1.6 in (4 cm) · 15 gal (55 L)75–82°F (24–28°C)
Scherer's hatchetfish
Carnegiella schereri
Freshwater1 in (2.6 cm) · 14 gal (54 L)73–82°F (23–28°C)
Common questions
What size tank does a Coral red pencilfish need?
A Coral red pencilfish needs at least 15 gal (55 L). It spends most of its time in the middle.
What temperature and pH does a Coral red pencilfish need?
A Coral red pencilfish is comfortable at 75–82°F (24–28°C) with a pH of 5.5–7.0.
How big does a Coral red pencilfish get?
An adult Coral red pencilfish reaches about 1.2 in (3 cm).
Does a Coral red pencilfish need a group?
Yes. A Coral red pencilfish is a shoaling species and should be kept in a group, not singly.
Related species
1.2 in (3 cm)15 gal (55 L)72–82°F (22–28°C)Barred pencilfish
Nannostomus espei
1.2 in (3 cm) · 15 gal (55 L)
1.1 in (2.8 cm)11 gal (40 L)75–82°F (24–28°C)Least pencilfish
Nannostomus minimus
1.1 in (2.8 cm) · 11 gal (40 L)
1.3 in (3.3 cm)15 gal (55 L)72–82°F (22–28°C)Three-lined pencilfish
Nannostomus trifasciatus
1.3 in (3.3 cm) · 15 gal (55 L)
1.4 in (3.5 cm)15 gal (55 L)72–82°F (22–28°C)Hockeystick pencilfish
Nannostomus eques
1.4 in (3.5 cm) · 15 gal (55 L)
1.4 in (3.5 cm)12 gal (45 L)75–82°F (24–28°C)Twostripe pencilfish
Nannostomus digrammus
1.4 in (3.5 cm) · 12 gal (45 L)
1 in (2.5 cm)12 gal (45 L)72–82°F (22–28°C)Dwarf pencilfish
Nannostomus marginatus
1 in (2.5 cm) · 12 gal (45 L)
