Taxonomy & naming
The rosy barb was described by Francis Hamilton in 1822 in his Account of the Fishes Found in the River Ganges and Its Branches, originally under the genus Cyprinus and long known as Barbus conchonius and then Puntius conchonius. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the valid combination as Pethia conchonius (Hamilton, 1822).
The genus Pethia was erected by Pethiyagoda, Meegaskumbura and Maduwage in 2012 as part of the dismantling of the old, oversized genus Puntius. Pethia holds a large group of small South Asian barbs distinguished by features of the body, scales and (often) a pattern of dark blotches; the rosy barb is one of its largest and most widespread members. The older combinations Barbus conchonius and Puntius conchonius remain common in aquarium and aquaculture literature, and the same fish is frequently encountered under those names.
Morphology
Pethia conchonius is a moderately deep-bodied barb that reaches up to about 5.5 in in total length, though aquarium fish are usually smaller. The body is silvery to brassy with a faint pinkish cast, typically marked with a dark blotch on the caudal peduncle and sometimes a fainter spot behind the gill cover. It lacks barbels.
The species is strongly sexually dimorphic, especially in breeding condition. The male becomes flushed with rose-red to coppery-red over the flanks, with the dorsal, anal and pelvic fins darkening to black at their edges; the female stays a plainer silver-olive and grows larger and deeper-bodied. Several ornamental strains have been developed, including long-finned and "neon" or intensified-colour forms selectively bred for the trade.
Habitat
The rosy barb is native to the fresh waters of northern South Asia — Afghanistan, Pakistan, India, Nepal and Bangladesh, with records reported from Myanmar. It occupies a broad range of habitats: streams, rivers, lakes, ponds and the still waters of the subtropical and tropical lowlands and lower hills.
FishBase gives a pH range of about 6–8 and a hardness of roughly 5–19 dGH, indicating tolerance of soft to moderately hard, neutral water. The species is notably tolerant of cooler temperatures compared with most tropical barbs, reflecting its occurrence in subtropical northern India and the Himalayan foothills, and this hardiness underlies both its success in the aquarium and its establishment as an introduced species in several warm and temperate regions worldwide.
Feeding
Pethia conchonius is an omnivore that feeds on small invertebrates, crustaceans, insect larvae, plant matter and detritus in the wild. It forages actively in open water and over the substrate, taking food at all levels.
In the aquarium it is among the easiest of fishes to feed, accepting flakes, pellets, and live and frozen foods without fuss. A diet that includes some live or frozen invertebrates and a vegetable component brings out the male's red breeding colour most strongly. Its bold, eager feeding makes it a reliable fish but means it can out-compete shyer tankmates at mealtimes.
Mating
Like barbs generally, the rosy barb is an egg-scattering, non-guarding spawner: there is no nest, no territory beyond the brief courtship, and no parental care. As a male comes into condition his red colour intensifies and he pursues females actively through vegetation.
Spawning takes place among fine-leaved plants, where the driving male and a gravid female press together and release eggs and milt. The adhesive eggs scatter and settle among the plants and on the bottom. The pairing lasts only for the act of spawning, after which the adults take no further interest in the eggs — indeed they will eat them readily.
Breeding
The rosy barb is easily bred and is a classic beginner's egg-layer. A separate spawning tank with soft, slightly acidic to neutral water, dense fine-leaved plants or a spawning mop, and a protective layer of marbles or mesh on the bottom is the usual approach, so that the scattered eggs fall beyond the reach of the hungry parents.
A well-conditioned female can release a few hundred eggs in a spawning. The adults are removed afterward to prevent predation on the eggs and fry. The eggs hatch in roughly one to two days at room-to-tropical temperatures, and the fry become free-swimming a few days later, when they are reared on infusoria followed by newly hatched brine shrimp. Because the species spawns readily and tolerates a wide temperature range, its ornamental strains are produced in large numbers.
In the aquarium
Pethia conchonius is a hardy, peaceful, active shoaling fish that has been kept since the early days of the hobby. It does best in a group of six or more, which settles its behaviour and encourages the males to display; kept in small numbers it can be a half-hearted fin-nipper, though it is far less prone to this than the tiger barb.
It suits a roomy aquarium with open swimming space and some planting, and is unusual among popular barbs in tolerating cooler water — it can be kept in an unheated room in temperate climates and is well suited to subtropical community tanks. Water chemistry is not critical within a broadly neutral, soft-to-moderately-hard range. Active mid-water companions of similar size make the best tankmates; very small or long-finned, slow fishes are less suitable.
Conservation
Pethia conchonius is listed as Least Concern (LC) on the IUCN Red List (assessed 2015). It is a widespread and adaptable species across a large range in northern South Asia, occurs in a wide variety of habitats, and is common where it is found, so it is not considered to be at risk at the species level.
Most rosy barbs in the aquarium trade are captive-bred, which further reduces any collection pressure on wild stocks. As with much of the South Asian freshwater fauna, localised threats from pollution, habitat modification and water extraction affect particular populations, but they do not currently threaten the species as a whole.