Taxonomy & naming
Betta ideii was formally described by Heok Hee Tan and Peter K. L. Ng in 2006 in a paper treating six new fighting-fish species from Borneo, published in Ichthyological Exploration of Freshwaters (vol. 17, no. 2, pages 97–114; type-series illustrated at fig. 7a–b). Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) lists the valid name as Betta ideii Tan & Ng, 2006; as a species described directly in Betta, no parenthetical authority is used.
The species belongs to the unimaculata group — one of the larger species assemblages within Betta — whose members share a paternal mouthbrooding reproductive strategy and tend toward greater body size and a more elongate build than the bubblenesting sibling groups. Other unimaculata-group species occur across Borneo and the Malay Peninsula, but B. ideii's range is restricted to the southernmost corner of the island.
The authority for the valid name is the Catalog of Fishes (Eschmeyer, CAS). The eponym (see Etymology) is the Japanese aquarist Takashige Idei, noted in Beolens et al. (2023) Eponym Dictionary of Fishes.
Morphology
Betta ideii is one of the larger members of the unimaculata group. FishBase records a maximum standard length of 3.5 in SL; Seriously Fish gives a size range of 3.5–4.5 in total length, indicating males can exceed 3.5 in and reach approximately 4.5 in. The body is elongate and somewhat laterally compressed, the typical profile of unimaculata-group species, which tend toward a more streamlined shape than the deep-bodied fighting forms of the splendens group.
Detailed meristic and colour data for B. ideii are sparse in the accessible literature. As with other members of the unimaculata group, wild coloration is generally more subdued than in display-bred domesticated species, and the fish shows its best colours in a breeding or territorial context. Sexually active males typically display richer colouration and elongated unpaired fins compared with females; fin development is the most reliable secondary character for field or captive sexing.
Habitat
Betta ideii is endemic to Kalimantan Selatan (South Kalimantan) province in Indonesian Borneo, with its range extending to Pulau Laut, a coastal island lying off the southeastern tip of Borneo. This is a restricted distribution even by the standards of wild bettas, which are frequently narrow endemics tied to single river drainages or peat-swamp systems.
Habitat data for this species in the primary literature are limited; the notes available suggest association with forested streams characterised by relatively clear, flowing water — a profile broadly consistent with other unimaculata-group bettas, which tend to inhabit shaded hill and foothill streams rather than the stagnant blackwater peat swamps favoured by many bubblenesting species. One reported water-chemistry figure is pH 5.5 at approximately 68 °F (68 °F), indicating cool, soft, and distinctly acidic conditions. As a labyrinth fish, B. ideii relies on its suprabranchial organ for atmospheric air-breathing and therefore requires unobstructed surface access regardless of dissolved oxygen levels.
Feeding
The natural diet of Betta ideii has not been formally studied, but is inferred from the ecology of the unimaculata group and the general feeding biology of wild bettas. Forested-stream bettas in this group are predominantly insectivorous, taking small invertebrates — aquatic larvae, terrestrial insects that fall to the water surface, small crustaceans — opportunistically. The relatively clear, flowing streams they occupy are richer in macroinvertebrate prey than the tannin-stained blackwater swamps that support many bubblenesting relatives.
In captivity, B. ideii accepts small live and frozen invertebrate foods readily: Daphnia, brine shrimp (Artemia nauplii and adults), bloodworm (chironomid larvae), and small earthworms are all reported as suitable. A diet weighted toward live and frozen foods maintains condition and encourages breeding behaviour. Dry foods may be accepted once the fish is established, but live prey remains the preferred staple for long-term health in this group.
Mating
Betta ideii is a paternal mouthbrooder, the reproductive mode shared by all members of the unimaculata group. Courtship follows the pattern typical of the group: the male courts a receptive female with displays of extended finnage and intensified colour, and the two fish engage in a nuptial embrace — the male wrapping his body around the female — during which eggs and sperm are released simultaneously.
After each embrace the fertilised eggs settle or are gathered by one or both fish; they are ultimately taken into the male's mouth. In unimaculata-group bettas the female sometimes assists initially, but brooding responsibility passes entirely to the male. Unlike bubblenesting Betta, no foam nest is constructed, and the spawning substrate is relatively unimportant; pairs will spawn over bare bottom or among plant roots. The male does not feed during the brooding period.
Breeding
Once the male has collected the clutch, he holds the fertilised eggs in his buccal cavity for the full incubation period — typically estimated at one to two weeks for unimaculata-group species, depending on temperature — without eating. At the end of incubation he releases free-swimming fry that are already past the most fragile larval stage and able to pursue small prey.
In captivity, breeders set up a species tank with soft, acidic water (around pH 5.5–6.5), a temperature in the low-to-mid 20s Celsius, and subdued lighting with ample cover. Heavily planted tanks or the provision of caves and leaf litter encourages natural behaviour. Once the male is visibly brooding — his throat and lower jaw noticeably distended — he should ideally be isolated or the female removed, as she may harass him. Fry are released at a size capable of taking newly hatched brine shrimp and micro-worms from the outset; the brooding male should be offered food immediately after releasing the fry, as he will be depleted.
In the aquarium
Betta ideii is an infrequently encountered species in the hobby, kept mainly by specialists in wild bettas and unimaculata-group enthusiasts. It demands conditions that reflect its cool, soft, acidic stream habitat: water with a pH around 5.5–6.5 and a temperature in the range of 68–75 °F (68–75 °F) — notably cooler than the paddy-water conditions often associated with domesticated B. splendens. Soft water with low conductivity is preferred, and the use of dried leaf litter (Indian almond leaves, oak leaves) to acidify the water and provide surface cover is widely recommended by keepers of wild unimaculata-group bettas.
As a labyrinth fish, unobstructed access to the water surface for atmospheric breathing is non-negotiable; a tight-fitting lid also prevents the jumping that this group is capable of. Filtration should provide gentle turnover without strong current — a sponge filter is standard. B. ideii is best kept as a pair or in a species-only setup; males of the unimaculata group will fight, and the species' relatively large size means it can easily outcompete or intimidate smaller tankmates. A tank of 23.5–31.5 in minimum length suits a pair, with floating plants and surface cover to give the brooding male security.
Conservation
Betta ideii has not been formally assessed by the IUCN Red List and is listed as Not Evaluated (NE). No published conservation assessments or population data are available for this species. Its restricted range — limited to southern Kalimantan Selatan and Pulau Laut — places it in a region of Borneo that has experienced substantial deforestation, peat drainage, and habitat conversion for agriculture and palm oil production, pressures that threaten stream-dependent fish species across the island.
Until a formal assessment is carried out, the conservation status of B. ideii remains uncertain. The combination of a small natural range, specialised habitat requirements, and ongoing land-use change in its part of Borneo suggests that precautionary concern is warranted. Hobbyist breeding programs that maintain captive stock independent of wild collection are one practical contribution specialist keepers can make in the absence of formal conservation action.