Care guide · Freshwater · fish

Blackwing hatchetfish

Carnegiella marthae

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The blackwing hatchetfish, Carnegiella marthae, is a small South American surface-dwelling fish in the family Gasteropelecidae. It occurs in the upper Orinoco and Rio Negro drainages. Hatchetfish species can be confused in trade, so check the whole body shape and markings rather than relying on a seller’s common name. This guide concerns C. marthae only; care information for the marbled or pygmy hatchetfish does not automatically apply. The fish is timid and is more secure when maintained in an appropriate shoal.

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At a glance

Temperature: 20–28 °C
pH: 4.0–7.0
GH: 1–10 °dGH
Aquarium length: At least 60 cm
Minimum aquarium volume: At least 54 L
Adult size: At least 2.8 cm
Group size: At least 10 individuals
Temperament: Peaceful

About this species

This is a genuinely small species. Seriously Fish reports 28 mm standard length, while FishBase gives a maximum close to 2.9 cm standard length and notes aquarium fish around 3.5 cm total length. Standard length ends at the base of the tail; total length includes the caudal fin. That difference in measurement explains the varied figures. Its deep, laterally compressed body and strong pectoral fins are characteristic of hatchetfishes, but these traits alone should not be used to distinguish closely related species.

Natural habitat

Carnegiella marthae is reported from the upper Orinoco and Rio Negro river basins in South America. It is associated with dark, acidic water and seasonally flooded forest margins with overhanging vegetation and leaf litter. Habitat descriptions are useful context, not a reason to make abrupt or extreme changes in a home aquarium. Provide wood, plants and floating cover for shelter while keeping a usable open area at the surface where the shoal can feed and swim.

Aquarium and layout

Seriously Fish recommends a tank base of at least 60 by 30 cm, about 54 litres, and a group of ten or more. This is a source-based minimum recommendation for a small shoaling fish, not a guarantee that every tank of those dimensions is suitable. A longer tank provides more surface area than a tall, narrow tank of equal volume. Use a secure, tightly fitting lid because startled hatchetfish may jump. Keep water movement gentle to moderate and check that all fish can reach food.

Water conditions

Published ranges are not identical. Seriously Fish lists 20–28°C, pH 4–7 and GH 1–10 dGH; FishBase gives narrower ecological reference values. These boundaries are not a target recipe and do not justify rapid pH adjustment or abrupt shifts. Keep temperature and water chemistry stable, measure reliably and acclimate new fish gradually. When possible, ask about the conditions in which the fish were held and avoid sudden changes during transfer.

Behaviour and group size

The blackwing hatchetfish is peaceful but small and retiring. Choose calm tankmates of comparable adult size, and consider the upper swimming zone: fast fish that continually take surface food can leave hatchetfish underfed. Large predators present an obvious risk, while persistently boisterous or fin-nipping species are unsuitable. Matching pH or temperature does not by itself prove compatibility. Assess shoaling needs, behaviour, adult size, food competition and available tank length together.

Feeding

In nature, Carnegiella marthae preys at or near the surface on small aquatic and terrestrial invertebrates and other zooplankton. Offer small portions suited to its mouth, such as Artemia nauplii, Daphnia, Moina or small grindal worms, live or frozen. It may accept appropriately sized dry foods, but ensure food remains available at the surface. Watch the whole shoal at feeding time and remove uneaten food before it affects water quality.

Breeding

The specialist sources reviewed do not provide a sufficiently detailed, species-specific home-aquarium breeding protocol. Habitat accounts mention seasonally flooded vegetation, but that does not establish an exact aquarium spawning method. Prioritise a stable shoal, suitable food and shelter. Treat breeding as uncertain, and do not infer guaranteed spawning, parental care or fry feeding requirements from accounts of other hatchetfish species.

Sources & review

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

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