Care guide · Freshwater · fish

Purple pencilfish

Nannostomus rubrocaudatus

No verified species photo available yet

Nannostomus rubrocaudatus is a tiny but vividly marked pencilfish from Peru. It was formally described only in 2009 and had earlier appeared in the trade under names such as “purple”. Its small body can be deceptive: males defend space and pursue rivals. A spacious, calm aquarium with cover and a proper group matters more than its size might suggest. After introduction, watch daily for feeding access, damaged fins and persistent chasing. Ask sellers about traceable origin and check that the group feeds actively.

Plan your aquarium →

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 species name refers to its red tail pattern. Adult males show red-purple colour between the dark lateral stripes; females are usually less intense. Colour changes with mood and social position, so a faded fish is not automatically female. Like other pencilfish, its pattern may pale markedly at night. Assess identity and condition under normal lighting and from several features.

Natural habitat

The documented distribution is Peru. Type specimens came from a shipment reportedly collected near Saramirisa on the Marañón, according to the importer. That locality detail rests on the trade statement. The specialist species profile says the natural habitat is unconfirmed, so a specific blackwater habitat should not be presented as established fact.

Aquarium and layout

Plan for an aquarium at least 90 cm long and about 81 litres for a mixed-sex group. This specialist recommendation mainly reflects the space rival males need. Dense plants, floating cover and wood can break sight lines while leaving open swimming routes. Use gentle filtration and a mature, stable aquarium; do not add the fish to a newly set-up tank.

Water conditions

The species-specific aquarium guidance is 24–28°C, pH 4–7 and GH 1–5 dGH. The broad pH range is not an invitation to move quickly between values: stability and appropriately low hardness matter. Test source water and make any adjustments gradually. The species account does not provide confirmed field measurements from the collection locality.

Behaviour and group size

It is generally peaceful toward other species but timid, and is a poor match for boisterous or much larger fish. Males spar territorially; in a small aquarium this can cause injuries. Prefer a group of ten or more, with enough space and cover to prevent one subordinate fish being singled out. Assess tankmates by adult size, temperament, swimming zone and feeding pressure; matching water values alone does not establish compatibility.

Feeding

This micropredator eats tiny invertebrates and zooplankton. Offer small portions of suitable live or frozen foods daily, such as Artemia nauplii, Moina or grindal worms. Fine dry foods may be accepted as part of a varied diet, but check that every fish can eat them. Remove leftovers; no packaged food is recommended until its particle size and use are verified.

Breeding

Aquarium breeding has been reported but is described as rare. A separate mature spawning tank with fine moss, fine-leaved plants or an appropriate spawning mop may collect eggs. Adults may eat eggs and larvae, so remove them in time or transfer the spawning medium. Newly hatched fry need microscopic food; use Artemia nauplii only once they are large enough. Success is not guaranteed.

Sources & review

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

View the sources