Taxonomy & naming
Danio kerri was described by Hugh McCormick Smith in 1931. The type locality is a pool associated with a hill stream on Koh Yao Noi, an island in Phang Nga Bay, southwestern Thailand. The Catalog of Fishes (Eschmeyer, CAS) recognises the valid combination as Danio kerri Smith, 1931.
The species was long placed in the genus Brachydanio — a genus used for the smaller, shorter-lateralled danios as opposed to the long-bodied Danio — and much of the older aquarium literature refers to it as Brachydanio kerri. Brachydanio has not been upheld as a distinct genus in modern systematic treatments, and all former Brachydanio species, including the zebra danio, now sit within an expanded Danio. The SeriouslyFish profile still uses the Brachydanio combination in its URL, reflecting this earlier usage.
Within Danio the blue danio belongs to the core group of true danios characterised by a bold horizontal stripe pattern, an upturned mouth and a small, active schooling lifestyle shared across South and Southeast Asia.
Morphology
The blue danio is a slender, laterally compressed cyprinid reaching approximately 1.5–2 in standard length, with a maximum total length around 2 in. The body plan is the classic danio form: a straight dorsal profile, a slightly upturned mouth adapted for surface feeding, and a pair of fine maxillary barbels.
The most striking feature is coloration. In fish from the northern part of the range — around Phang Nga and Phuket — the midlateral stripe is a vivid, iridescent blue, contrasting sharply with the pale belly and olive dorsum. Populations from the southern end of the range, around Ko Lanta and into Langkawi, tend toward a more yellowish or golden lateral stripe, a geographic variation that can cause confusion in the aquarium trade where mixed-origin stock is common. A second, less prominent stripe may be visible below the main lateral stripe depending on angle of light and fish condition.
Sexual dimorphism is subtle but present: males are slimmer with a more intense coloration; females are deeper-bellied and noticeably rounded when gravid.
Habitat
Danio kerri has a restricted natural range centred on the islands and coastal mainland of southwestern Thailand and the extreme northwest of Peninsular Malaysia. Confirmed localities include Koh Yao Noi (the type locality), Phuket, Ko Lanta, and Langkawi island. The species is associated with small streams, pools and hill-stream drainages in this island-and-coastal-peninsula landscape.
FishBase notes that the type material was collected from a pool in a hill stream, and the species' close association with the karst and granite island terrain of the Andaman Sea fringe defines both its natural character and its narrow range. Water conditions in the hobby and from SeriouslyFish data indicate the species prefers cool to warm subtropical water: temperatures of 64–79 °F, a pH of 6.5–7.5, and soft to moderately hard water (roughly 36–215 ppm). These parameters reflect the generally soft, slightly acidic drainages typical of the Thai-Malay peninsula rather than the harder waters of large river mainstreams.
Feeding
In the wild Danio kerri is primarily insectivorous, taking aquatic and terrestrial insects and their larvae from the surface and water column — a diet characteristic of active, surface-oriented danios. Its upturned mouth is well suited to intercepting insects that fall onto the water surface or drift in the current.
In the aquarium it is an eager and undemanding feeder that accepts a wide range of prepared foods including quality flakes and micro-pellets. Condition, colour and breeding readiness are best maintained by supplementing the staple diet with live or frozen invertebrates: bloodworm, daphnia and brine shrimp (Artemia nauplii) are all suitable. Like other danios it is a fast, competitive feeder and should be watched to ensure quieter tankmates receive their share.
Mating
Danio kerri is an egg-scattering, non-guarding spawner. There is no nest construction, no pair bond and no parental investment beyond the act of spawning itself. Courtship follows the typical danio pattern: a ripe male pursues a gravid female energetically through vegetation or open water, pressing close alongside her and stimulating the release of eggs and milt simultaneously.
Spawning is most readily triggered by a small rise in water temperature and the introduction of fresh, slightly cooler water — mimicking the onset of the seasonal rains that flood and refresh the shallow habitats these fish occupy in the wild. Adults will readily consume their own eggs once spawning is complete, so separation of eggs from adults is necessary in a breeding setup.
Breeding
The blue danio is straightforward to breed in captivity. A separate spawning tank is recommended — a small aquarium with a mesh or fine-leaved plants (Java moss, hornwort or similar) covering the bottom gives the scattered eggs somewhere to settle out of the parents' reach. Well-conditioned adults are introduced in the evening; spawning usually occurs at dawn with the onset of light.
Eggs are adhesive and scattered over plants or substrate; a female may deposit several hundred eggs in a session. The adults should be removed after spawning. At 75–79 °F the eggs hatch in approximately 24–48 hours, and the larvae become free-swimming within a further two to three days. First foods are infusoria or commercial fry foods of equivalent fine particle size, followed by newly hatched brine shrimp nauplii as the fry grow. Growth is relatively rapid and the young fish can be sexed at a few months of age. Breeding mode: egg-scatterer; no parental care.
In the aquarium
The blue danio is a peaceful, active schooling fish suited to the community aquarium. It should be kept in groups of at least six — larger shoals display bolder colour and more natural behaviour, reducing the low-level chasing that isolated individuals may direct at tankmates. Its modest size and peaceful temperament make it compatible with other small, active species of similar water-parameter requirements.
A longer tank with open swimming space in the upper and middle water column, a soft planted backdrop, and gentle filtration suits the species well. Water should be maintained within the parameters it experiences in the wild: 64–79 °F, pH 6.5–7.5, and soft to moderately hard. The tolerance for cooler water at the lower end of that range means the blue danio can be kept without supplemental heating in a temperate room during the warmer months, though stability is more important than any particular value. It is not demanding, but a monotonous diet and cramped quarters will dull both colour and behaviour.
Conservation
The IUCN Red List assesses Danio kerri as Least Concern. Despite its geographically restricted range — centred on a handful of islands and coastal localities in Phang Nga Bay and the Andaman coast — the species was not considered to be in significant decline at the time of assessment, and populations in its core island habitats retain some protection by virtue of the limited human modification of small island stream systems compared to major river valleys.
That said, the species' island-focused distribution makes it inherently vulnerable to localised threats: deforestation for tourism development, agricultural runoff, and water extraction from the small stream systems it inhabits all represent plausible pressures on specific localities. Aquarium specimens in the trade are largely captive-bred, reducing direct collection pressure on wild populations.