Taxonomy & naming
Sundadanio goblinus was formally described in 2011 by Kevin W. Conway, Maurice Kottelat and Heok Hui Tan. The genus Sundadanio was erected by Kottelat and Witte in 1999 to accommodate a small set of diminutive, laterally compressed cyprinids endemic to the peat swamp forests of Borneo and Sumatra — fish previously shuffled between Rasbora and Microrasbora but now recognised as a distinct lineage within the Danioninae. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Sundadanio goblinus Conway, Kottelat & Tan, 2011.
The species epithet is drawn from the Anglo-French goblin, a mischievous creature of small stature and peculiar appearance. According to the original description, the name alludes simultaneously to the species' diminutive size and to something vaguely impish in the proportions of its head and eye — a self-aware piece of nomenclature from a group of authors who have described many miniature Sundaic cyprinids. There are no widely established English vernacular names in the hobby literature.
Morphology
With a maximum recorded standard length of 0.5 in, Sundadanio goblinus is among the smallest cyprinids in the world. Like its congeners it is laterally compressed, deep-bodied relative to its length, and bears the proportionally large eye typical of micro-danios adapted to the dim, tea-coloured waters of peat swamps. The lateral line is incomplete or absent, as is characteristic of the genus.
The coloration in life involves the intense, metallic iridescence common to Sundadanio — typically blue-green to blue-violet flanks that shift with angle of light, in a pattern of horizontal lines or a diffuse sheen depending on lighting conditions. Specific details of pattern differ between congeners; published colour descriptions of S. goblinus itself are sparse in the accessible hobbyist literature, and the original description provides the authoritative account.
Habitat
Sundadanio goblinus is endemic to coastal peat swamp forests of Sumatra, Indonesia. Peat swamp forests in the Sundaic lowlands occupy shallow basins where thick accumulations of partially decomposed plant matter create an extraordinary water chemistry: extremely soft (very low ionic content), acidic (pH commonly 3.5–5.5), and rich in dissolved humic and fulvic acids from decomposing vegetation. The result is the characteristic dark, tea-coloured or black water of the region, with very low conductivity and negligible mineral hardness.
These forests are benthopelagic freshwater environments with dense canopy cover, abundant submerged roots and leaf litter, and virtually no current. Light penetration is low. Water temperatures in lowland Sumatra are relatively stable and warm year-round. This is a habitat of extreme chemistry and extreme biological specificity — the fish that have evolved there, including the Sundadanio, are rarely found outside it.
Feeding
Sundadanio goblinus is a micropredator of the open water and water column within the peat swamp, taking small invertebrates appropriate to its minute size: zooplankton, tiny aquatic insect larvae, copepods and other micro-crustaceans. As with other members of the genus, the upturned or terminal small mouth suits picking prey from the water column and the surface film rather than from the substrate.
In the aquarium, congeners of similar size accept live and frozen micro-foods such as newly hatched brine shrimp nauplii, micro-worm, daphnia and copepods. Dry foods in suitably small particle size (crushed micro-pellets or fine flake) are also taken by established fish, but live and frozen items are important for conditioning.
Mating
Sundadanio goblinus is an egg-scattering, non-guarding spawner — the standard reproductive mode for Danioninae cyprinids. There is no nest, no bubble-nest, and no mouthbrooding. When ready to spawn, a male pursues a gravid female actively through fine-leaved plants, moss or roots, the pair pressing briefly together while eggs and milt are released simultaneously.
The adhesive eggs fall among plant material or onto the substrate after release, and the adults show no further interest in them. On the contrary, given the chance, adults will consume the eggs. There is no pair bond beyond the moment of spawning.
Breeding
Breeding Sundadanio goblinus in captivity demands close attention to water chemistry. Because the species is adapted to the extreme soft-acid conditions of Sumatran peat swamps, successful spawning almost certainly requires very soft, acidic water — pH well below neutral, very low hardness, and the presence of tannins from driftwood, peat or dried leaf litter. A species tank is strongly preferable to a community arrangement given the fish's tiny size.
As with close congeners, a densely planted spawning vessel with a sand or fine-gravel substrate and an abundance of fine moss or leaf litter provides cover for the eggs and reduces the risk of immediate predation by the adults. Eggs are scattered among the plants and substrate and should hatch within a few days at appropriate temperatures. The larvae are tiny and require infusoria or the finest live foods at first feeding. Data on captive breeding of S. goblinus specifically are sparse in the accessible literature, and the approach is largely inferred from congeners.
In the aquarium
Sundadanio goblinus is a specialist fish demanding soft, acidic, tannin-stained water that replicates peat swamp conditions. Hardness should be negligible and pH well below 7.0 — values in the range of 4.5–6.5 are appropriate, following the chemistry of its native habitat. A blackwater setup with driftwood, dried leaf litter and peat filtration is ideal. Standard tap water treated only for chlorine is almost certainly unsuitable; reverse osmosis water remineralised to very low GH and acidified is the realistic approach.
At 0.5 in maximum length, this is one of the smallest fish available in the hobby and must be kept only with similarly sized, peaceful species or in a species-only tank. A group of eight or more is recommended — schooling is natural behaviour and a larger group distributes the fish's activity and reduces individual stress. Tank size need not be large, but water quality must be impeccable and chemistry stable. This is not a fish for a beginner or a general community tank.
Conservation
Sundadanio goblinus is assessed as Endangered (EN) on the IUCN Red List, with the assessment formalised on 4 January 2019 under criteria B1ab(i,ii,iii). The primary threat is the conversion of Sumatran coastal peat swamp forests to agricultural land — principally oil palm and acacia pulpwood plantations. This process has accelerated dramatically since the late twentieth century and has eliminated or severely degraded vast areas of lowland peat swamp across Sumatra and Borneo.
The peat swamp forest habitat is not merely the physical location of S. goblinus — it generates the precise water chemistry (extreme acidity, softness, tannin load) to which the species is physiologically adapted. Habitat loss therefore does not leave the fish to persist in degraded remnants; drainage and disturbance of the peat fundamentally alter the hydrology and chemistry that define the species' niche. Research into the freshwater biodiversity of Sundaland's peat swamps has documented this dynamic in detail, noting that endemic miniature cyprinids such as the Sundadanio are among the most extinction-vulnerable groups as peatland conversion continues.