Taxonomy & naming
Arnoldichthys spilopterus was described in 1909 by the Belgian-British ichthyologist George Albert Boulenger, originally under the genus Alestopsis before being placed in its own monotypic genus, Arnoldichthys, named in honour of Johann Paul Arnold, a German aquarist and early importer of tropical fish. Eschmeyer's Catalog of Fishes lists the valid combination as Arnoldichthys spilopterus (Boulenger, 1909), within the family Alestidae — the African tetras, a family that also includes the larger Alestes and Brycinus and the smaller Micralestes and Phenacogrammus (Congo tetra).
As the only described species in its genus, Arnoldichthys occupies a somewhat isolated taxonomic position among the African tetras, distinguished from its relatives by a combination of its very large scales, distinctive eye colouration and fin markings. The specific epithet spilopterus derives from the Greek for "spotted fin," referring to the dark blotch and patterning on the fins that helps distinguish the species.
Morphology
Arnoldichthys spilopterus is a laterally compressed, moderately deep-bodied characin reaching around 3 in in total length, making it one of the larger fish regularly kept as a "tetra" in the aquarium trade. Its most distinctive features are unusually large, coin-like scales along the flanks, a vivid red iris that gives the species its common name, and an iridescent greenish-gold to blue-green sheen along the upper body that shifts with the light. A dark, often diffuse blotch sits behind the gill cover, and the anal fin is edged in black and white, a combination of features that makes the species easy to identify at a glance.
Sexual dimorphism is present but subtle: males tend to be slightly slimmer and more intensely coloured, with a somewhat more pointed dorsal fin, while females are deeper-bodied, particularly when carrying eggs, and slightly less vivid in colouration.
Habitat
The African red-eyed tetra is native to the Niger Delta and neighbouring coastal river systems of southern Nigeria and adjacent West African drainages, including tributaries such as the Ogun River. It inhabits slow-flowing, often heavily vegetated lowland rivers, streams and floodplain habitats typical of the West African coastal forest and delta zone, frequently in soft, slightly acidic, tannin-tinted water beneath riparian forest cover, though it also tolerates somewhat harder, more open-water conditions in parts of its range.
The Niger Delta is one of the most industrially developed and heavily impacted river systems in Africa, owing to decades of petroleum extraction and associated infrastructure; oil spills, gas flaring and general habitat degradation are well-documented, long-standing problems across the region that directly affect the aquatic habitats this and many other Niger Delta fish depend on.
Feeding
Arnoldichthys spilopterus is an omnivore in the wild, taking a broad diet of insects, other small invertebrates, plant matter and fallen fruit or seeds from overhanging vegetation, consistent with its habit of foraging in well-vegetated, slow-flowing water. In the aquarium it is an easy and unfussy feeder, readily accepting high-quality flake and pellet foods, and does particularly well with the addition of live or frozen foods such as bloodworm, brine shrimp and daphnia, along with occasional vegetable matter to round out its diet.
Mating
As with most characins, Arnoldichthys spilopterus is an egg-scattering open spawner with no parental care. In the aquarium, spawning is typically triggered by soft, slightly acidic, warm water and can be encouraged with a fine-leaved plant thicket or spawning mop as an egg-catching substrate; a pair or small group will scatter adhesive eggs among the vegetation over the course of an active, energetic spawning bout, usually in the early morning.
Breeding
Breeding Arnoldichthys spilopterus in captivity is achievable but not routine for beginners, in part because adults are enthusiastic egg predators and eggs must be protected from the parents, typically by using a mesh spawning grid or by removing the adults promptly after spawning. Soft, acidic water (a pH around 6.0–6.5 and low hardness) with a temperature in the upper 20s Celsius favours successful spawning and egg development. Eggs hatch within roughly 24–36 hours, and the tiny fry become free-swimming a few days later, requiring infusoria or very fine commercial fry food initially before graduating to newly hatched brine shrimp.
In the aquarium
The African red-eyed tetra is a lively, active schooling fish best kept in groups of six or more in a reasonably spacious aquarium — its adult size and activity level make it unsuitable for very small tanks. It is generally peaceful toward similarly sized tankmates but can be a boisterous, fin-nipping presence toward slow-moving or long-finned species if kept in too small a group or too cramped a space, so a larger shoal and adequate swimming room help keep its energy directed at its own kind rather than at tankmates.
It adapts to a fairly wide range of water chemistry in captivity, though it shows its best colour in soft, slightly acidic, well-filtered water with subdued lighting and dark substrate, conditions that echo the tannin-stained forest streams of its native range. A tightly fitting lid is recommended, as the species is an active jumper. Given a well-planted tank with open swimming space at the front, a shoal of these fish makes a genuinely striking display, their red eyes and shifting green-gold flanks catching the light as they move.
Conservation
The CARES Preservation Program lists Arnoldichthys spilopterus at the Endangered (CEN) priority level, a striking classification for a species that remains a familiar and regularly available aquarium fish. The listing reflects the severe and well-documented environmental degradation of its native Niger Delta habitat: decades of oil extraction, pipeline leaks, gas flaring, and general industrial pollution have caused extensive damage to the wetland and river ecosystems of the delta, independent of the species' continued presence in the aquarium trade, much of which is now supplied by farmed and captive-bred stock rather than wild collection.
The African red-eyed tetra is a useful reminder that popularity in the aquarium hobby and security in the wild are not the same thing — a species can remain common and easy to buy while its native habitat quietly deteriorates around it. Maintaining robust captive-bred lines, and supporting broader efforts to address pollution and habitat loss across the Niger Delta, both have a role to play in the species' longer-term outlook.