Hatchetfish

1 Species Overview

Also known as the Swallow Fish, Chest-Knife Fish, or Sternoptyx, the hatchetfish is a group of unusually shaped small freshwater fishes. Its body is extremely laterally compressed with a greatly protruding belly, giving it the silhouette of an ancient stone axe—hence the name.

Aliases: Hatchetfish, Swallow Fish, Chest-Knife Fish, Sternoptyx

Binomial: Sternoptyx diaphana

Latin name: Sternoptyx diaphana

Species: Sternoptyx diaphana

Common name: Marine Hatchetfish

Family: Sternoptychidae (hatchetfishes)

Distribution: Pacific Ocean, Indian Ocean—tropical, subtropical, and some temperate regions; also the South China Sea and East China Sea

Class: Actinopterygii

Phylum: Chordata

Kingdom: Animalia

Genus: Sternoptyx

Order: Salmoniformes

2 Appearance & Physical Traits

Hatchetfish are exceptionally timid and highly sensitive. When startled, they react with nervous, darting bursts of speed and frequently leap clear of the water surface—so keepers must accommodate this behavioral quirk when designing the aquarium layout.

Many varieties exist, all sharing the same basic plan: small freshwater fishes with extraordinary body shapes. Their bodies are extremely compressed; viewed from above they appear as a narrow straight line, with a flat back and a dramatically protruding belly that gives a side profile resembling an ancient stone axe—hence the name. Eyes are large; the mouth is superior (upturned), perfectly adapted for surface feeding. The dorsal fin is set far back; the caudal fin is long with rays of equal length; the tail is forked. Pectoral fins are remarkably large, and during swimming the fish appears to glide like a bird in flight—hence the nickname ‘Swallow.’ Most species have plain, understated coloration with transparent fins. Depending on variety, body length ranges from 2.5 to 9 cm. Sexual dimorphism is minimal; during breeding season, a distended belly indicates a gravid female, but otherwise sexes are hard to tell apart.

The two hatchetfish types most commonly seen in aquariums are the Coffee Hatchetfish (also called Black Hatchetfish, ~4 cm, semi-transparent dark brown body) and the Marble Hatchetfish (also called Yin-Yang Hatchetfish, ~4 cm, with diagonal camouflage patterning in black, orange, and brown).

3 Habitat & Living Environment

When keeping hatchetfish, the single most important factor is water quality. While not extremely difficult to care for, their sensitive and timid nature demands careful attention to environmental design.

Hatchetfish prefer weakly acidic soft water, with optimal temperatures between 22–28°C. They tolerate abrupt environmental changes poorly and often succumb to illness shortly after purchase due to ‘transport shock.’ Avoid frequent water changes and keep lighting subdued. They feed almost exclusively on small, active live foods—if you can condition them to accept prepared diets, your chances of success improve significantly. Given the considerable difficulty involved, few hobbyists choose hatchetfish unless they enjoy a genuine challenge.

The hatchetfish’s defining characteristic is timidity: when frightened, they dart nervously and frequently jump, so maximize surface area whenever possible. Keeping them in schools enhances their sense of security—never keep fewer than six individuals. Given the risk of losses during initial acclimation, purchasing around ten at once is wise.

Most keepers start with small or juvenile specimens because they are easier to obtain and the juvenile cuteness is appealing. However, many overlook their ‘growth potential’ and the problems that arise when adults outgrow inadequate space! If husbandry goes wrong and a hatchetfish dies young, you won’t see stunted growth or skeletal deformities from crowding—but if you’re serious about keeping them, research their expected adult size and plan adequate space from day one. Start with a larger-than-necessary tank even for juveniles; while the tank may look ’empty’ at first, generous space promotes faster, healthier growth. Species that grow rapidly when young tend to become proportionally larger, stronger, and healthier adults.

4 Feeding Guidelines

For hatchetfish, the old adage ‘eat well, grow strong’ applies in full force. Their substantial bodies demand nutritionally balanced rations. Once you’ve set up a suitable habitat, how should you select healthy, appropriate foods?

In nature, hatchetfish hunt small fish and insects exclusively—a diet that provides excellent nutritional balance when combined with constant foraging and ample exercise plus zero space constraints, allowing uniform, robust growth. In the aquarium, hatchetfish rarely lack food, but keepers who fixate on a single type of live prey create nutritional imbalances. For balanced nutrition, rotate among different live foods—for example, in a typical month: one feeding of small fish, two of shrimp, and one of insects.