Taxonomy & naming
Danio tweediei was described by Brittan in 1956. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises it as a valid species distinct from Danio albolineatus (Blyth, 1860), the widespread pearl danio with which it was historically synonymised. The separation matters because albolineatus and tweediei occupy different drainage systems and show consistent morphological differences, even though both carry iridescent, pearl-like flanks that invited confusion.
The genus Danio — the true danios — is a group of small, horizontally patterned or iridescent schooling cyprinids centred on South and Southeast Asia. The literature contains older combinations placing this and related species in the genus Brachydanio, a name that enjoyed some use for the smaller, short-bodied danio species; current usage follows Catalog of Fishes in retaining tweediei within Danio. FishBase records the species under this combination. Older hobby and trade literature may list specimens as Brachydanio albolineata or conflate them with Danio albolineatus entirely.
Morphology
Danio tweediei is a small, elongate, laterally compressed cyprinid. FishBase gives a maximum recorded standard length of 2.5 in (with a typical common length around 1.5 in SL) and a maximum total length of 4 in, making it somewhat larger than some of its congeners. The flanks carry the characteristic iridescent sheen of the pearl danio group — fine horizontal lines or a diffuse metallic lustre rather than the bold discrete stripes of Danio rerio. The mouth is slightly upturned and the body is slim and built for active, open-water schooling.
Sexual dimorphism is modest: females become noticeably deeper-bodied and more rounded in the belly when gravid; males are slimmer and more streamlined, as is typical for the genus. No information on colour differences between the sexes is recorded in available sources beyond this general pattern.
Habitat
Danio tweediei inhabits flowing hill streams and clear running waters across its range, with records from the Irrawaddy, Salween, Mekong and Mae Khlong drainages in Myanmar and Laos, and from Peninsular Malaysia including Penang, as well as Sumatra. FishBase characterises it as a surface-active fish of running waters, typically found near the surface in clear, well-oxygenated streams.
The water chemistry reported from these habitats spans a broad range: FishBase records a pH of 6.0–8.0 and a hardness of 5–19 dH, and a temperature range of 68–77 °F. Seriously Fish notes a somewhat wider temperature window of 61–77 °F, consistent with the cooler hill-stream character of some localities. Substrates in clear hill streams are typically rocky or gravelly with clean, well-oxygenated flow — quite different from the still, muddy paddies and backwaters favoured by lowland danio species.
Feeding
Danio tweediei is an insectivore and micropredator. FishBase records its diet as exogenous insects and zooplankton — a diet consistent with an active surface-feeding fish in well-oxygenated streams where terrestrial and aquatic invertebrates are available at and near the water surface.
In the aquarium, the species accepts a wide range of prepared foods once acclimatised. Small-particle flakes and micropellets form a practical staple, supplemented with live and frozen invertebrates such as daphnia, artemia and bloodworm to maintain condition and encourage natural feeding behaviour. Its active, top-to-mid-water feeding style means it competes effectively at mealtimes in a well-stocked community tank.
Mating
Like all true danios, Danio tweediei is an egg-scattering, non-guarding spawner. There is no nest, no pair bond, and no parental care of any kind. In a ripe group, a receptive male drives a gravid female through vegetation or over open substrate, pressing close alongside her as eggs and milt are released together.
The stimulus for spawning in hill-stream danios is typically a combination of well-oxygenated, clean water, slight temperature fluctuation, and the presence of fine-leaved plants or a suitable substrate over which eggs can be scattered. In the wild, spawning likely increases during seasonal shifts associated with rainfall and stream flow in the drainages these fish inhabit.
Breeding
Danio tweediei breeds in the manner typical for the genus: it is an egg-scatterer that distributes adhesive eggs over plants, moss or gravel and provides no parental care. Seriously Fish notes that fry appear spontaneously in densely planted aquaria, reflecting how readily the species spawns when conditions are suitable.
A dedicated breeding setup uses a shallow tank furnished with fine-leaved plants or spawning mops, or a layer of marbles over the bottom to allow eggs to fall beyond the reach of the adults. A well-conditioned group is introduced; spawning typically follows within a day or two. Adults must be removed after spawning to prevent predation on the eggs. Eggs hatch in approximately two to three days, and the fry become free-swimming a few days later, initially taking infusoria before graduating to newly hatched brine shrimp and fine powdered foods.
In the aquarium
Danio tweediei is a hardy and active schooling fish that does well in a standard community aquarium. It should be kept in groups of at least six — the social nature of danios means solitary or paired fish become stressed and may pester tankmates. A longer tank with open swimming space, some planting, and a gentle current to simulate its stream-dwelling origins suits it well.
Its reported temperature range of 61–77 °F makes it notably flexible for a Southeast Asian species, tolerating room-temperature conditions that are comfortable for temperate fish yet still suitable for many tropical tankmates. Water chemistry is equally accommodating across its documented pH range of 6.0–8.0 and a moderate hardness range. It is undemanding to feed and accepts flakes and small pellets alongside live and frozen foods. The species is sometimes mislabelled in the trade as Danio albolineatus or sold simply as "pearl danio"; buyers seeking tweediei specifically should confirm provenance where possible.
Conservation
Danio tweediei has not been formally assessed by the IUCN Red List as of the time of writing, and carries no conservation status. The species' historical conflation with Danio albolineatus — which is itself assessed as Least Concern — means it has not been evaluated independently as a valid species with its own range, population trend and threats.
Its wide distribution across Myanmar, Laos, Peninsular Malaysia and Sumatra and its occurrence in multiple large drainage systems (Irrawaddy, Salween, Mekong, Mae Khlong) suggest it is not immediately at risk, but the hill-stream and clear-water habitats it favours are sensitive to deforestation, sedimentation, agricultural runoff and water extraction — pressures that have degraded stream quality across much of Southeast Asia. A formal IUCN assessment using current range data from valid-species records would be a useful first step toward understanding its conservation position.