Taxonomy & naming
Sundadanio rubellus was formally described by Conway, Kottelat and Tan in 2011 as part of a comprehensive taxonomic revision of the genus Sundadanio, which erected or redefined several species endemic to peat-swamp and blackwater habitats across the Sundaland region of Borneo and Sumatra. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Sundadanio rubellus Conway, Kottelat & Tan, 2011. GBIF corroborates the authority and places the species within one of seven Sundadanio species recognised in the 2011 revision.
The genus Sundadanio sits within the family Danionidae (broadly the danios and their relatives), and members of Sundadanio have historically been traded and sometimes described under rasbora-like vernacular names — "neon red rasbora" persists in the hobby — though the genus is not closely allied to Rasbora proper. The specific epithet rubellus is Latin for reddish or somewhat red, a direct reference to the vivid red-orange fin and body coloration of adult males.
Morphology
Sundadanio rubellus is a diminutive, laterally compressed fish reaching a recorded maximum of 0.5 in standard length, placing it among the tiniest members of its family. The body is slender and translucent, typical of micro-danios that inhabit dimly lit, tannin-stained waters. Adult males develop striking red-orange pigmentation in the fins and on portions of the flanks, giving them the vivid appearance that earned the species its common name and its Latin epithet.
Sexual dimorphism is evident: males are smaller and more intensely coloured, displaying the red-orange hues used in courtship, while females are fuller-bodied, rounder in the abdomen when carrying eggs, and generally plainer in coloration. The species is demersal in habit, foraging in and among the lower water column and substrate of its peat-swamp environment.
Habitat
The species is endemic to the southern Kapuas River delta in West Kalimantan, Indonesian Borneo. Its recorded type locality is within the peat swamp forests of this delta system. The Kapuas River is the longest river in Borneo, and its lower delta and associated peat swamp forest complex is a zone of exceptionally high biodiversity and endemism among freshwater fishes.
Sundadanio rubellus inhabits slow-flowing to still, highly acidic blackwater — the classic soft, tannin-stained water produced as decaying peat and leaf litter leach humic acids into the waterways of lowland Sundaland peat swamp forest. Care guides record water parameters of pH 5.0–6.5, hardness of GH 1–5 (very soft), and temperatures of 72–79 °F. This is a warm, seasonally stable environment very different from the cool hill streams of northern Borneo or the alkaline plateau lakes of Myanmar; any aquarium keeping this fish must replicate the soft, acid chemistry of its peat-swamp home.
Feeding
Like most micro-danios, Sundadanio rubellus is a micropredator and opportunistic omnivore in the wild, subsisting on tiny aquatic invertebrates, zooplankton and algae found among the detritus and fine vegetation of its peat-swamp habitat. Its small mouth constrains it to appropriately miniature prey.
In the aquarium the species adapts readily to prepared foods of suitably small particle size — quality micro-pellets and commercial flake crushed fine — but thrives best when live and frozen foods such as newly hatched brine shrimp, microworms and daphnia form a significant portion of the diet. Varied feeding maintains the vivid red-orange coloration of males and supports the reproductive condition of females.
Mating
Sundadanio rubellus is an egg-scattering, non-guarding spawner with no parental investment beyond the act of spawning itself. Courtship involves an active male pursuing a gravid female through vegetation and fine-structured cover; the pair release eggs and milt together in brief, pressing bursts among plants, moss or fine substrate.
There is no pair bond and no territorial guarding of eggs or fry. The adhesive eggs settle among plants and substrate where the adults will consume them if given the opportunity. Stable, species-appropriate water chemistry — soft, acidic, warm — and the availability of live foods appear to be important triggers for spawning readiness in this species.
Breeding
Breeding Sundadanio rubellus in the aquarium demands replication of its peat-swamp home conditions. Acidic, very soft water (pH 5.0–6.5, GH 1–5) with temperatures around 75–79 °F, subdued lighting, tannin staining from peat or Indian almond leaves, and dense fine-leaved vegetation or java moss provide the environment in which the species is most likely to spawn.
The species is an egg-scatterer; adults place no parental care on eggs or fry and will eat both if accessible. A separate, well-planted breeding tank or the use of fine mesh over the bottom to protect eggs is the standard approach. Conditioning with live foods — brine shrimp nauplii, microworms, daphnia — brings both sexes into spawning condition. Fry are tiny at hatching and require infusoria or similarly microscopic first foods before graduating to newly hatched brine shrimp. Given the species' threatened wild status, successful captive breeding is of conservation significance.
In the aquarium
Sundadanio rubellus is a specialist species best suited to experienced keepers who can maintain the strict water chemistry it requires. Acidic, very soft blackwater conditions (pH 5.0–6.5, GH 1–5, 72–79 °F) are non-negotiable; in hard, alkaline tap water the species declines rapidly. Peat filtration, Indian almond leaves or peat extract, and a reverse-osmosis or rain-water base provide the appropriate chemistry. Subdued lighting and dense vegetation round out a setup that mimics the shaded, leaf-littered peat-swamp floor.
At under 1 in, it is one of the smallest fishes in the hobby and must be kept with appropriately sized, peaceful companions — or better, in a species tank. It is a shoaling fish and should be kept in groups of at least eight to twelve to express natural behaviour and reduce individual stress. Its diminutive size makes it vulnerable to even small, fast tankmates. Because its wild habitat is under severe threat and its range is narrow, specimens in the trade may include wild-caught fish; captive-bred stock is preferable where available.
Conservation
The IUCN Red List assessed Sundadanio rubellus as Near Threatened in January 2019 (Assessment ID: T91075306A91075410). The species' extent of occurrence is estimated at approximately 12,559 mi², a figure that falls within the threshold criteria for Vulnerable under criterion B, though population trends and subpopulation fragmentation are the primary basis for the Near Threatened rating rather than the raw area alone.
The primary driver of threat is habitat loss: the peat swamp forests of West Kalimantan and Borneo more broadly have been subjected to extensive drainage, burning and conversion to palm oil plantations and agricultural land. Peat swamp forest is among the most rapidly disappearing biomes in Southeast Asia, and the specialist blackwater fish fauna it harbours — including Sundadanio rubellus and several other narrowly endemic micro-danios — has nowhere else to go. Industrial-scale land clearance, fire, and hydrological disruption of peat catchments continue to reduce and fragment the habitat. Sustained deforestation pressure means the species' situation may deteriorate toward Vulnerable or higher threat categories without meaningful habitat protection in the southern Kapuas delta region.