Common hatchetfish

Geographical Origin & Biotope:

Endemic to a massive, sprawling geographical range across the vast expanse of the lower Amazon basin in Brazil and the Guianas (Guyana, Suriname, and French Guiana). Gasteropelecus sternicla (universally known as the Common or Silver Hatchetfish) natively colonizes densely vegetated, sluggish, deeply flooded jungle streams, completely stagnant backwaters, and oxbow lakes. They live their entire existence exclusively at the absolute surface of the water column, hunting terrestrial insects that fall from the rainforest canopy.

Taxonomy & Morphology:

Scientifically classified within the Gasteropelecidae family (the true freshwater hatchetfishes), it is a spectacular, bizarrely specialized top-dweller. Morphologically, fully mature adults reach roughly 6.0 to 6.5 centimeters (2.3-2.5 inches) in length. It possesses a highly compressed, paper-thin, incredibly deep "hatchet" shaped body. The critical anatomical feature is an immensely oversized, heavily muscled pectoral girdle powering massive, rigid, wing-like pectoral fins. This unique musculature allows them to launch themselves entirely out of the water to catch flying insects.

Social Behavior:

They are exceptionally peaceful, intensely nervous, and strictly obligate shoaling surface-dwellers. They absolutely MUST be kept in a sizable group (minimum 6, but 10+ is spectacular). They possess zero interest in the middle or bottom levels of the aquarium. They spend their entire lives hovering silently, in perfectly synchronized schools, directly below the surface tension. They are incredibly skittish; sudden vibrations or turning on bright lights without warning will trigger a violent, synchronized explosion of the entire school into the air.

Coloration & Sexual Dimorphism:

Sexual dimorphism is virtually non-existent; mature females may appear slightly plumper in the upper body when heavily laden with eggs, but they are nearly impossible to sex accurately. The coloration of the Common Hatchetfish is beautifully understated and provides absolute camouflage from submerged predators looking upward: the entire body is a gleaming, highly reflective metallic silver or pale, icy platinum. It lacks the dark mottling of the Spotted Hatchetfish, instead featuring a faint, solid dark lateral line running horizontally across its upper flank.

Tank Setup:

The aquarium architecture MUST flawlessly accommodate their strictly top-dwelling nature and phenomenal jumping ability. A minimum 120-liter (30-gallon) tank (at least 80 cm long) is required. The absolute most critical, life-saving requirement is a heavy, 100% secure, perfectly tight-fitting glass lid with NO gaps whatsoever. They are evolved to literally fly out of the water; an open tank is a guaranteed death sentence. Massive amounts of floating plants (like Amazon Frogbit) are absolutely mandatory to provide overhead security and diffuse lighting.

Diet & Feeding:

They are highly specialized, exclusive surface micro-predators (carnivores/insectivores). Their deeply upturned mouths are evolved perfectly for snatching insects off the surface tension. They physically cannot swim down to eat food off the substrate. You MUST provide a diet that stays strictly on the surface for extended periods. Daily offerings of high-quality floating flakes and freeze-dried bloodworms are mandatory. The absolute ultimate food for this species is live, wingless fruit flies (Drosophila) dropped directly onto the water surface.

Water Quality:

Originating from deeply shaded, pristine jungle streams, they demand highly stable, immaculate, soft, and slightly acidic blackwater. They thrive in warm tropical temperatures (24-28°C / 75-82°F). Crucially, they require soft water (GH 2-10) and a slightly acidic to neutral pH (6.0 - 7.0). They possess absolute zero tolerance for dissolved organic waste; rigorous weekly water changes are mandatory. The water flow MUST be incredibly gentle at the surface; strong surface agitation or wavemakers will violently exhaust them and cause immense physiological stress.

Compatibility & Tankmates:

Compatibility is excellent, provided tankmates are exceptionally peaceful and occupy entirely different zones of the aquarium. They are the perfect top-dweller for a dedicated, peaceful Amazonian community. Excellent companions include peaceful mid-water Tetras (Cardinal Tetras, Rummy-nose Tetras), peaceful Dwarf Cichlids (Apistogramma), and peaceful bottom-dwelling Corydoras or Plecos. They MUST NEVER be housed with aggressive, fast-swimming, or surface-feeding fish (like large Danios, aggressive Barbs) that will ruthlessly outcompete them.

Aquarium Breeding:

Breeding in the home aquarium is exceptionally rare, poorly documented, and incredibly difficult to trigger. It requires an absolute pristine, heavily planted blackwater setup and massive conditioning with live insects (specifically wingless fruit flies). Breeding is believed to be triggered by a sudden drop in atmospheric pressure and massive, soft water changes. The adults engage in a frantic courtship at the surface, scattering tiny eggs among floating roots. The microscopic fry reside exclusively at the surface and require infusoria to survive.

Risks & Diseases:

The absolute greatest physical risk is lethal leaping; they are evolved specifically to fly out of the water to escape predators. They WILL launch themselves out of the tank through the tiniest gap in the lid, making a 100% secure, heavy cover absolutely mandatory. The second major risk is starvation caused by offering sinking foods; their upturned mouths make it impossible to eat off the bottom. Finally, sudden noises, vibrations, or violently switching on the tank lights will trigger extreme panic, often causing them to crash into the lid.