Silver hatchetfish

Geographical Origin and Habitat: Thoracocharax stellatus, commonly known as the silver hatchetfish or spot-fin hatchetfish, is native to South America. It is widely distributed in the river basins of the Amazon, Orinoco, and Paraguay-Paraná, covering countries such as Brazil, Colombia, Venezuela, Paraguay, and Argentina. It inhabits slow-moving rivers, forest streams, oxbow lakes, and flooded areas of the tropical forest, remaining strictly in the top few centimeters of the water column under the cover of floating plants.

Taxonomy and Morphology: It belongs to the class Actinopterygii, order Characiformes, family Gasteropelecidae, and genus Thoracocharax. It is the largest member of the hatchetfish family, capable of reaching a maximum length of 7-9 centimeters (2.7 to 3.5 inches). The morphology is extraordinarily specialized: the body is extremely laterally compressed, with a deep, arched sternal keel housing powerful pectoral muscles. The pectoral fins are long and strong, resembling wings.

Social Behavior: It is a peaceful, timid, and schooling fish that must be kept in groups of at least 6-8 individuals to reduce stress and promote natural behavior. It spends 100% of its time near the water surface. If startled by sudden movements or changes in light, it reacts by performing spectacular leaps out of the water, using its strong pectoral fins to glide over the surface.

Coloration and Sexual Dimorphism: The body presents a highly brilliant, mirror-like silvery coloration with golden and greenish reflections depending on the angle of light. A dark longitudinal line runs along the sides from the operculum to the base of the caudal fin. A characteristic dark spot is present on the dorsal fin. Sexual dimorphism is minimal: mature females are slightly larger and show a rounder, more swollen abdomen than males.

Aquarium Setup: Requires a long aquarium with a minimum length of 100 cm (approx. 39 inches) to allow school swimming. The crucial aspect of the setup is the water surface, which must be partially covered with floating plants (such as Pistia stratiotes, Salvinia, or Limnobium) to provide shade and security. It is absolutely mandatory to install a tight-fitting, gap-free lid, as these fish will jump out at the slightest startle.

Diet and Feeding: It is a specialized surface predator with an upward-pointing mouth. In the aquarium, it feeds exclusively on food floating on the water surface, completely ignoring anything that falls to the bottom. The diet must include small live insects such as fruit flies (drosophila), micro-crickets, high-quality floating flakes or small pellets, and floating frozen or freeze-dried foods like white or black mosquito larvae.

Water Quality: Requires soft, slightly acidic or neutral water. Optimal parameters are: temperature between 23°C and 28°C (73°F - 82°F), pH between 6.0 and 7.0 (can tolerate up to 7.5), and hardness GH between 4 and 12. Filtration must be efficient to keep the water clear and free of pollutants (nitrates < 15 ppm), but the water flow at the surface must be kept weak to moderate so as not to hinder their peaceful swimming.

Compatibility and Cohabitation: An ideal guest for community aquariums dedicated to the South American biotope. Being peaceful and occupying only the surface zone, it can be combined with fish that inhabit the middle and lower zones of the aquarium, such as small characins (neons, cardinals), dwarf cichlids (Apistogramma), corydoras, and small loricariids. Avoid aggressive surface fish or large, boisterous tankmates.

Aquarium Breeding: Breeding in home aquariums is extremely difficult and very rarely reported. It requires a dedicated breeding tank with extremely soft and acidic water (pH 5.5-6.0) filtered over peat, and a dense layer of floating plants. Spawning occurs among the plant roots, and the eggs are adhesive. Parents must be removed immediately after spawning. Fry are extremely small and must be fed with infusoria.

Risks and Diseases: The primary lethal risk is jumping out of the aquarium due to the lack of an adequate cover. Because of their timid nature, they are particularly prone to contracting Ichthyophthirius multifiliis (Ich, or white spot disease) in case of sudden temperature drops during water changes. It is recommended to heat new water to the same temperature as the tank before adding it.