Coral red pencilfish

Nannostomus mortenthaleri

Coral-red colour between dark bands changes with social rank

Dominant males can be intensely red between the upper dark bands. Subordinate males fade, and females may also become reddish; redness alone does not tell sex.

Adult length: Up to 2.9 cm (standard length, without the tail fin)
Water temperature: 24–28 °C
pH: 4–7
Hardness: 1–5 dGH

Native range: The Nanay basin in Loreto, Peru, and the Tigre system farther west.

Natural habitat: Lives in sluggish tributaries, small rivers and swamps near vegetation, submerged wood and leaf litter, including blackwater flooded forests and floodplain lakes.

Social hierarchies can cause prolonged harassment, injury or death. Dense planting does not compensate for insufficient space. Use a mature aquarium with stable water conditions.

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Appearance and identification

Compare the red upper interstripe of adult males with the compact, three-banded dwarf pencilfish. The similar N. rubrocaudatus differs in head markings and fins, not overall redness.

Same-species behaviour

Keep ten or more, or ten to twenty in a larger, well-structured aquarium. Watch subordinate fish.

Space requirements

At least a 90 × 30 cm base for a mixed-sex group, with dense shelter and broken sight lines.

Diet and feeding

Offer fine dry foods and small frozen or live foods; check that subordinate individuals receive food.

Compatibility limits

Watch chasing, feeding competition and size. Take particular care with similar-looking pencilfishes.

Aquarium environment

Plants and roots provide retreats and interrupt sight lines. Retain swimming room and gentle flow.

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