Taxonomy & naming
Betta miniopinna was described by Tan Heok Hui and Tan Ee Khoo in 1994, with the holotype (ZRC 32504) deposited at the Zoological Reference Collection, National University of Singapore. The type locality is a swamp forest site near Pasir Segiling at the Tanjong Bintan end of northern Pulau Bintan, Riau Islands, Indonesia, at approximately 1°10′N 104°30′E. Eschmeyer's Catalog of Fishes recognises the valid name as Betta miniopinna Tan & Tan, 1994.
The species is placed in the coccina group (sometimes called the red fighting fish group), a cluster of small, blackwater-adapted bettas distributed across the Sundaland region, including species such as B. coccina, B. rutilans, B. tussyae, and B. persephone. Members of this group share small adult size, subdued to richly coloured body patterns, and a preference for extremely soft, acidic water. Within the group, B. miniopinna is distinguished primarily by the intensity and extent of red colouration on the pelvic fins, which gives the species its name.
Morphology
Betta miniopinna is a very small fish, reaching approximately 1–1.5 in standard length (SL) in adults, placing it among the smaller members of the genus. The body form is typical of the coccina group: slender and slightly laterally compressed, with relatively large eyes and the upturned mouth characteristic of a surface-oriented predator. The pelvic fins are elongate and filamentous in males.
The most conspicuous feature is the pelvic fins, which in males display an intense cinnabar-red or orange-red colouration — the feature commemorated in the species name. Overall body colouration is brownish to reddish-brown, often with an iridescent blue-green sheen on the flanks in certain light, and some individuals show faint vertical banding. Males are more intensely coloured and carry more elaborate finnage than females, which are generally plainer and shorter-finned. The species shares the labyrinth organ (suprabranchial air-breathing organ) common to all bettas, which is visible as a slight bulge behind the head.
Habitat
Betta miniopinna is a stenotopic specialist of peat swamp forest — one of the most chemically extreme freshwater habitats in the world. On Pulau Bintan, wild populations occupy shaded, still or barely moving waters overlying deep accumulations of partially decomposed plant material. The substrate is soft mud and leaf litter; the water is stained dark brown by dissolved tannins and humic acids leaching from peat, and light penetration is minimal.
Water chemistry in these habitats is exceptionally soft and acidic: pH values below 4.0 have been recorded in undisturbed Bintan peat swamps, with conductivity near zero and virtually no dissolved minerals. For aquarium purposes, conditions of pH 4.0–6.0, negligible hardness, and temperatures of 72–82 °F bracket the species' natural tolerance. Being a labyrinth fish, B. miniopinna breathes atmospheric air via its suprabranchial organ, which allows it to thrive in the warm, oxygen-depleted water typical of stagnant peat pools — an adaptation shared across the family but particularly relevant in habitats where dissolved oxygen can be near zero.
The microhabitat within the swamp is characterised by submerged and floating leaf litter, root tangles, and overhanging vegetation that create the dim, sheltered conditions the species requires. Open, exposed, or well-lit water appears to be avoided.
Feeding
Like other members of the coccina group, Betta miniopinna is a carnivore specialised on small invertebrates. In the wild, the diet consists of small aquatic invertebrates — insect larvae, microcrustaceans, and similar prey — as well as small terrestrial invertebrates that fall onto the water surface. The upturned mouth is well-adapted to taking prey at or near the water surface.
In the aquarium, the species accepts small live and frozen foods readily: Daphnia, micro-worms, banana worms, small blackworms, and newly hatched brine shrimp are all suitable. Dried foods are often refused or taken reluctantly, and the species generally does better on a varied live or frozen diet. Given its very small size, prey items must be appropriately fine; large foods are ignored or cannot be processed.
Mating
Betta miniopinna is a bubble-nester — the male builds a small raft of mucus-bound air bubbles at the water surface, typically beneath a large floating or overhanging leaf or another piece of cover. Males are territorial around the nest site and display to females with erect fins and intensified colouration, particularly showing off the red pelvic fins.
Courtship follows the pattern typical of bubble-nesting bettas: the male performs lateral displays near a receptive female, and after prolonged courtship the pair engage in a spawning embrace below or at the nest. The male wraps around the female in a nuptial clasp; eggs and milt are released simultaneously. Males in captivity have been observed to use large-leafed plants and similar cover to anchor or shelter the nest.
Breeding
Spawning produces a small clutch — typically fewer than 20 eggs — which the male gathers and blows into the bubble nest immediately after release. The male assumes sole responsibility for nest maintenance and guarding, repairing damaged sections of the nest and retrieving any eggs or early fry that fall. The female should be removed after spawning to prevent aggression from the nest-guarding male.
Eggs hatch within a few days at typical temperatures, and the fry initially hang vertically from the nest, completing yolk absorption before becoming free-swimming. Once free-swimming, the fry are tiny and require very fine first foods — infusoria, micro-worms, or commercial liquid fry food — before graduating to newly hatched brine shrimp nauplii. The male continues to guard the fry for a short period after they become free-swimming, though he should be removed once the fry are consistently mobile. Rearing water must be very soft and acidic to reflect the species' chemistry requirements; hard or alkaline water at the fry stage can be fatal.
In the aquarium
Betta miniopinna is an advanced-level species demanding very specific conditions that mirror its peat swamp origin. A small, heavily planted species tank is appropriate: a volume of 10–15 US gal is sufficient for a pair or small group, provided water chemistry is carefully managed. Filtration must be gentle — powerful flow is incompatible with the still-water biotope and can prevent bubble-nest formation or destroy it — and the tank must have a tight-fitting lid to maintain warm, humid air above the surface for the labyrinth organ to function.
Water chemistry is the critical parameter: pH should be maintained in the range 4.0–6.0, hardness as low as possible (ideally near zero GH/KH), and temperature between 22 and 82 °F. Reverse-osmosis water conditioned with peat or commercial blackwater extract is the standard approach. Tannin-rich décor — dried leaves, driftwood, botanicals — recreates the biotope and helps maintain the acid chemistry. Standard tap water is unsuitable without extensive treatment.
Males may be kept with females in a well-planted tank with sufficient cover, but two males will fight. Given its rarity in both the wild and the trade, responsible husbandry includes attempting to breed the species and sharing offspring within the hobby network to reduce demand for wild-caught specimens. The species is not suitable for community tanks; tankmates, if any, should be restricted to other small, non-aggressive species adapted to blackwater conditions.
Conservation
Betta miniopinna is assessed as Critically Endangered (CR) on the IUCN Red List, under criterion A2c, reflecting a severe and ongoing decline in the extent and quality of its peat swamp forest habitat. The assessment dates to 1996 and reflects conditions on Pulau Bintan, where peat swamp forests have been extensively cleared for agriculture, plantation forestry, and development. Bintan's peat swamps have been severely reduced since the species was described, and the species' range was already extremely limited at the time of description.
As a species endemic to a single island with a highly specialised, shrinking habitat, B. miniopinna faces existential pressure with little resilience to further loss. No large outlying populations exist to buffer local decline. Collection for the ornamental fish trade represents an additional pressure on already small wild populations, though the species is rarely encountered in commercial trade.
Conservation measures for the species depend primarily on the protection and restoration of Bintan's remaining peat swamp forest. In the aquarium hobby, keeping and breeding B. miniopinna from established captive lineages rather than seeking wild-caught individuals is the most direct contribution an aquarist can make. The species is maintained by specialist betta keepers and some conservation-oriented hobbyist groups, but captive populations are small and genetically limited.