Taxonomy & naming
Pethia nigrofasciata was described by the German-British ichthyologist Albert Günther in 1868, originally placed in the genus Puntius — a large, heterogeneous assemblage of South and Southeast Asian barbs that served as a catch-all for most of the group for well over a century. The Catalog of Fishes (Eschmeyer, California Academy of Sciences), the authority for valid names, records the parenthetical authority (Günther, 1868), reflecting the transfer to the current genus.
The genus Pethia was erected by Pethiyagoda, Meegaskumbura and Maduwage in 2012 as part of a major revision of the old Puntius complex. The revision split Puntius sensu lato into several natural groupings based on morphology and biogeography; Pethia accommodates a cluster of small, typically banded, South Asian species — many of them Sri Lankan endemics. The name Pethia is drawn from a Sinhala vernacular name for small barbs, anchoring the genus in the island's own naturalist tradition. Older literature and aquarium records may list this fish as Puntius nigrofasciatus, Barbus nigrofasciatus, or Systomus nigrofasciatus; all are synonyms under the current Catalog of Fishes placement.
Morphology
The black ruby barb is a small, moderately deep-bodied and laterally compressed cyprinid. FishBase records a maximum total length of 2.5 in for females; males are slightly smaller, typically reaching around 2.5 in TL. The standard length of adults in the trade tends to run 2 in.
The most striking feature is the male's breeding coloration. The head and anterior third of the body become a rich reddish-purple, deepening toward black on the gill cover and nape. The flanks carry three to four broad, dark vertical bars against a paler background; the fins are dark-edged or partially blackened. Females are considerably less colourful — they retain the barred pattern but remain olive-brown to yellowish rather than flushing red, and their bodies are deeper and more rounded when gravid. The contrast between the sexes is pronounced enough that males in peak colour look like a different species from resting females.
Habitat
The black ruby barb is endemic to southwestern Sri Lanka, occurring in the wet zone from the Kelani River basin in the north to the Nilwala River basin in the south. These drainages descend from the central highlands through a belt of lowland tropical rainforest that receives 2,000–3,0.5 in of annual rainfall — among the highest on the island. Wild populations have been recorded at elevations up to roughly 984 ft.
Within this range the species occupies clear, cool, shaded forest streams running over gravel and sand, typically with marginal vegetation, overhanging tree roots, leaf litter and twigs along the banks. The water is soft and slightly acidic, with low conductivity reflecting the granite and metamorphic catchments: FishBase records a temperature range of 72–79 °F, a pH of approximately 6.0–6.5, and a hardness of 5–12 dH. Seriously Fish characterises the wild habitat as low-conductivity, slightly acidic, cool-water streams under forest canopy — conditions closely tied to intact riparian forest. As that forest has been cleared for tea, rubber and paddy cultivation, suitable stream habitat has contracted sharply.
Feeding
Pethia nigrofasciata is an omnivore with a strong herbivorous component. In the wild it grazes on filamentous algae and diatoms, picks through organic detritus and leaf matter, and supplements its diet with small invertebrates — insect larvae, worms, crustaceans and zooplankton — taken from the stream bed and water column.
In the aquarium it is undemanding and accepts good-quality flake and small granular foods readily. Vegetable matter — spirulina-based foods, blanched leafy greens, algae wafers — should form a regular part of the diet given the wild preference for algae and detritus. Live and frozen foods such as bloodworm, brine shrimp and daphnia improve condition and intensify male coloration, which is particularly valuable before a breeding attempt. The species poses no threat to robust aquatic plants.
Mating
Like all barbs, Pethia nigrofasciata is an egg-scattering, non-guarding spawner. There is no nest, no bubble-nest and no pair bond beyond the moment of spawning. A conditioned male pursues a gravid female through shallow water among marginal weeds or fine-leaved plants, pressing against her flanks until eggs and milt are released simultaneously. His head and anterior body flush their deepest reddish-purple during this period, and multiple males may compete to follow a single receptive female.
The adhesive eggs fall among plant stems or settle onto the substrate, where — without any guarding instinct — they are at immediate risk of being eaten by the spawning adults. Hobbyists commonly separate adults from the eggs as soon as spawning is complete, or provide dense planting and substrate cover to give the eggs a chance in a community setting.
Breeding
The black ruby barb breeds readily in captivity and has been commercially farm-raised for decades, a circumstance that has kept it in the trade despite the export prohibition on wild Sri Lankan fish. A typical spawning setup uses a shallow tank with soft, slightly acidic water, fine-leaved plants or spawning mops over a substrate of fine gravel or a mesh layer to protect eggs from the parents above.
Females may scatter over 100 adhesive eggs in a session; FishBase notes the eggs hatch in one to two days at aquarium temperatures. The larvae are initially very small and require infusoria or rotifers as a first food before graduating to newly hatched brine shrimp. Adults show no parental care and will consume eggs and fry without hesitation, so removing them after spawning is standard practice. Farm-raised stock is generally more adaptable in its water chemistry tolerance than wild fish, accepting pH values from roughly 5.5 to 7.5 and a wider temperature band.
In the aquarium
The black ruby barb is a peaceful, schooling species that is best kept in groups of at least six to ten individuals. In sufficient numbers the fish form a cohesive shoal and display their most intense colour; in smaller groups they become subdued and may nip at slower tankmates. Their temperament is far gentler than many barbs, and they are commonly recommended for community tanks with other peaceful mid-water species of comparable size.
Wild-type water conditions — cool (68–79 °F), soft, slightly acidic — suit the species best and produce the most vivid male coloration, but farm-raised fish will generally adapt to neutral, moderately hard tap water provided it is clean and well-oxygenated. Bright, harsh lighting tends to wash out colour; a tank with some floating plants or overhead cover, driftwood, and a dark substrate shows this fish at its best. Good filtration maintaining strong oxygenation — reflecting the clear, well-aerated forest stream habitat — is beneficial. The species is not demanding at the table and accepts quality flake, frozen and vegetable foods without difficulty.
Conservation
Pethia nigrofasciata is assessed as Vulnerable (VU) on the IUCN Red List, most recently evaluated on 07 August 2019 under criteria B1ab(iii,v)+2ab(iii,v). The assessment reflects a severely restricted range — limited to two river basins in southwestern Sri Lanka — combined with continuing decline in habitat quality and extent. The IUCN notes that more than 35% of forest cover in the wet zone was lost between 1990 and 2005, and that by 2006 only approximately 4.6% of the island's original forest remained. Conversion to tea, rubber and paddy cultivation, together with the effects of civil conflict on land use, has fragmented and degraded the riparian forest streams the species depends on.
The export of wild Pethia nigrofasciata from Sri Lanka is currently prohibited by national law, a regulation aimed at curtailing the collection pressure that once coloured the trade. However, the IUCN assessment notes that illegal wild collection continues and that selective collection for the most colourful males may be altering the genetic structure of remaining wild populations. The species' survival in the aquarium hobby is now effectively sustained by farm-raised stock — produced in significant quantities in Asia and Eastern Europe — rather than by wild harvest. Protecting and restoring the wet-zone forest streams of the Kelani and Nilwala basins remains essential for the long-term security of wild populations.