Gold-lined Knifefish / Longtail

Geographical Origin and Habitat: Continent-wide distribution across the Amazon, Orinoco, and upper Paraná systems. Strictly benthic, lurking heavily camouflaged among dense sunken roots, mudbanks, and flooded forests in mid-to-large rivers.

Taxonomy and Morphology: A true giant of the Sternopygidae family (related to the tiny Glass Knifefish). Growth is relentless: reaches 50-60 cm (20-24 inches) in aquaria, sometimes larger in the wild. Very thick, muscular body compared to the razor-thin smaller knives. Lacks dorsal and caudal fins; the tail tapers off into an exceptionally long, thin 'whip' (hence macrurus or longtail). Navigates and hunts using a continuous weak electrical pulse (electrolocation).

Social Behavior: Purely nocturnal and highly benthic. Very shy as a juvenile, hiding all day, but becomes much bolder as a massive adult, dominating the tank floor at night. Its behavior changes as it matures: from tolerant to SAVAGELY AGGRESSIVE toward its own kind or other knifefish species. Electrical interference causes pure rage, resulting in biting and torn flesh until the weaker fish dies.

Coloration and Sexual Dimorphism: Subtle but breathtaking. The laterally compressed body is a dark, matte olive-black or brown velvet. The signature mark is a vivid, striking gold or creamy-white line tracing perfectly horizontally from the gills straight to the tip of the tail. Usually sports a large, black, white-rimmed 'eye-spot' behind the gills. Females are thicker in the abdomen.

Aquarium Setup: A true half-meter monster requiring a MASSIVE tank (minimum 200 cm / 6-7 feet length). A FINE SAND substrate is mandatory—they lack protective scales and will quickly develop fatal flesh-eating fungal infections if housed over sharp gravel or rough lava rocks. You MUST provide an opaque, dark PVC pipe or a massive hollow stump for it to retreat into during daylight hours. Dimly lit tanks prevent psychological stress.

Diet and Feeding: Nocturnal Carnivore. Voracious appetite as an adult. Feed substantial, meaty items using tongs directly near its cave at dusk: whole thawed shrimp (shell removed), mussels, jumbo earthworms, and smelt (silversides). Easily trained onto massive, sinking carnivore pellets. Will completely ignore all vegetable matter. Any small or sleeping fish will be eaten during the night.

Water Quality: Extremely vulnerable to tank chemistry due to its 'naked' skin. Copper medications, malachite green, and formalin will kill it rapidly. Requires pristine water via massive biological filtration. pH 6.0-7.5. Temp 23-28°C (73-82°F). Gentle bottom current preferred.

Compatibility and Cohabitation: MUST BE KEPT AS A SOLITARY KNIFEFISH. Zero tolerance for conspecifics or other electric fish. Best housed in peaceful 'Monster Fish' setups: large Oscars, Uaru, Severums, giant armored Plecos, or Freshwater Stingrays. It will systematically hunt down and swallow small/medium fish (Tetras, Corydoras) in the dark.

Aquarium Breeding: Undocumented in the hobby due to the sheer violence of keeping two adults together in confined volumes.

Risks and Diseases: 1. LETHAL RADAR WARS: Keeping two adults in the same tank guarantees a fight to the death due to electrical jamming. 2. Fatal chemical burns from cheap, copper-based aquarium medications. 3. Fungal/Bacterial ulcerations of the belly from rough substrates.