Taxonomy & naming
Horadandia atukorali was described by P. E. P. Deraniyagala in 1943, with the type locality recorded at Attidiya near Colombo, Sri Lanka. The species name honours Mr Atukorala, the Sri Lankan naturalist who brought it to Deraniyagala's attention. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the valid combination as Horadandia atukorali Deraniyagala, 1943.
The monotypic genus Horadandia was erected to receive this species, and it has been subject to occasional revision. For a period the Indian populations were treated as a distinct species or subspecies, but a 2013 revision reassessed the situation and the fish from Kerala, Tamil Nadu and Puducherry are now included within H. atukorali, the Sri Lankan endemic designation having been relaxed. Horadandia sits within the Danioninae and is closely allied to the broader micro-danio assemblage — miniature cyprinids of the lowland tropics of South and Southeast Asia — though it is morphologically distinctive enough to warrant its own genus.
Morphology
The green carplet is exceptionally small: FishBase records a maximum standard length of 1 in, with commonly observed fish often closer to 0.5 in SL, making it one of the tiniest cyprinids in the hobby. The body is slender and laterally compressed, with a slightly upturned mouth suited to surface and mid-water feeding. The fin counts are modest — 10 dorsal soft rays, 8 anal soft rays — and the vertebral count falls between 24 and 26.
Live colouration is the fish's main ornamental feature: the flanks carry an iridescent greenish or bluish sheen that shifts with angle of light, most intense along the lateral midline. The belly is pale silvery-white and the fins largely clear. There is no bold stripe pattern of the type seen in Danio or Devario; instead the fish relies on its metallic sheen and schooling behaviour for display. Sexual dimorphism is subtle: females become visibly rounder in the belly when gravid; males are marginally slimmer and may show slightly more intense lateral iridescence.
Habitat
The green carplet is a fish of tropical coastal lowlands. Its native habitat spans western Sri Lanka — the coastal plain centred on Colombo, Gampaha, Veyangoda and Puttalam districts — and the adjacent Indian coastline of Kerala, Tamil Nadu and Puducherry. Across this range it occupies still and slow-moving water: swamps, seasonal pools, weedy ponds and the sheltered margins and irrigation channels of rice paddies, typically in shallow water with abundant emergent and floating vegetation.
Water conditions reflect the lowland tropical setting. SeriouslyFish records a preferred temperature range of 68–79 °F (FishBase gives 75–79 °F as optimal), with soft to moderately hard water (36–215 ppm hardness) and a pH of 5.5–7.5, leaning toward the acidic end where swamp waters are tannin-influenced. The fish is not a fast-stream species; it favours the stillest, most vegetated microhabitats within an already sluggish landscape.
Feeding
In nature the green carplet feeds on the fine-particulate fare available in shallow, vegetated, slow-moving water: small zooplankton, microcrustaceans, insect larvae and organic detritus caught at or near the surface and in the water column. Its small mouth size constrains prey to correspondingly minute items.
In the aquarium it accepts small live and frozen foods readily — Daphnia, Moina, Artemia nauplii and micro-worms are all suitable and improve condition and colour. Quality fine-grained flakes or micro-pellets are accepted once fish have settled. Given the fish's diminutive mouth, standard community dry foods should be checked for particle size; finely crushed foods or dedicated nano-fish formulations are preferable. Feeding twice daily with small portions prevents fouling in the nano tanks this species typically inhabits.
Mating
Horadandia atukorali is an egg-scattering, non-guarding spawner. There is no pair bond, no nest construction and no parental care — in keeping with the reproductive strategy shared by the overwhelming majority of danionine cyprinids. Spawning follows the typical micro-danio pattern: a conditioned male pursues a gravid female through fine-leaved vegetation or near the substrate, and eggs and milt are released together in brief driving chases.
The eggs are adhesive and come to rest among plant stems, moss or bottom debris, where the adults — if not removed — will readily consume them. In the wild, spawning likely corresponds to the onset of the wet season and rising water levels, when newly flooded shallow margins offer both spawning substrate and protection for the scattered clutch.
Breeding
Breeding the green carplet in captivity is achievable with attention to scale and water quality. A small, dedicated spawning vessel with a fine-mesh net or java moss layer to protect eggs from the adults is the standard approach. Soft, slightly acidic, warm water (around 77 °F) and good conditioning on live or frozen foods encourage readiness. Because this is an exceptionally small fish, a modest group of six to eight adults in a five to ten litre vessel is appropriate.
SeriouslyFish reports an incubation period of 47–54 hours at tropical temperatures, after which the larvae begin to absorb their yolk sacs. First foods must be microscopic: Paramecium or rotifers are essential at this stage, transitioning to freshly hatched Artemia nauplii as the fry grow. Growth is slow relative to larger danionine species. Adults should be removed after spawning to prevent predation on the eggs. Maintaining good water quality through small, regular water changes is critical during the fry-rearing phase.
In the aquarium
The green carplet is a specialist fish, not a typical community species. Its tiny size (commonly 0.5 in) makes it incompatible with most conventional community tanks, where it risks being outcompeted at feeding or eaten outright by larger fish. It is best kept in a dedicated nano aquarium — 5.5–10 US gal — or a carefully curated micro-fish community with similarly scaled species such as Boraras, Sundadanio or small Microdevario.
A group of at least ten is strongly recommended: SeriouslyFish notes that smaller groups become nervous and retiring. A heavily planted tank with floating leaf cover, driftwood or dried leaf litter, and dim or dappled lighting replicates the shaded swamp edge habitat and brings out the fish's best colours. Gentle filtration (sponge filters work well) keeps current low and avoids sweeping small fish. Water parameters of 68–79 °F, pH 5.5–7.5 and soft to moderately hard water are appropriate. The species is not commercially common; acquiring it usually means sourcing from specialist importers or hobbyist breeders.
Conservation
Horadandia atukorali is assessed as Vulnerable (VU) on the IUCN Red List, assessed in August 2019 under version 2022.2. The assessment reflects both the species' restricted geographical range — confined to coastal western Sri Lanka and a narrow band of southern India — and continuing declines in its swamp and paddy-field habitat from drainage, agricultural expansion and water quality degradation.
The coastal lowlands of Sri Lanka have experienced substantial habitat loss as swamps are drained for agriculture or urban development, and the species' dependence on slow, vegetated, shallow water makes it particularly sensitive to these changes. Indian populations in Kerala, Tamil Nadu and Puducherry face analogous pressures from the intensification of lowland rice cultivation and the channelisation or pollution of coastal freshwaters. The green carplet has a small footprint in the aquarium trade, and any captive interest in the species is better served by working with captive-bred stock and participating in records of captive breeding success, which can support ex-situ conservation of this threatened micro-cyprinid.