Taxonomy & naming
Parosphromenus opallios was described by Maurice Kottelat and Peter K.L. Ng in 2005 in The Raffles Bulletin of Zoology, based on specimens collected from the Arut River drainage in the Pangkalanbun area of Central Kalimantan (Kalimantan Tengah), Indonesian Borneo. The type series includes a holotype and paratypes, with one paratype (CMK 16355) measuring 1 in standard length. The valid name, as recorded in Eschmeyer's Catalog of Fishes, is Parosphromenus opallios Kottelat & Ng, 2005.
The species is placed in the genus Parosphromenus Bleeker, 1879, the licorice gouramis, a group of small labyrinth fishes (family Osphronemidae, subfamily Macropodusinae) endemic to soft, acidic lowland waters of the Malay Peninsula, Sumatra, and Borneo. Within the genus it was initially compared most closely to Parosphromenus deissneri, from which it is distinguished by coloration and fin-ray formulae. The specific epithet opallios is from the Greek for opal, alluding to the opalescent sheen of the unpaired fins in living males — a direct, observable reference to the fish's appearance rather than a patronym.
Morphology
Parosphromenus opallios is a slender, small-bodied fish, with males reaching approximately 1 in total length (roughly 1 in standard length based on type specimens; FishBase records males at 1 in SL). Like all Parosphromenus it is laterally compressed with rounded, softly rayed fins and the extended, filamentous tips on the caudal and anal fins that characterise displaying males in many species of the genus.
The most striking feature of breeding males is the coloration of the unpaired fins: the dorsal and anal fins are dark (blackish) proximally, grading to red distally, with a conspicuous black blotch at the posterior base of the dorsal fin. The caudal fin shares this dark-to-red gradient. The body itself is brownish to dark, often with indistinct dark lateral banding or mottling that provides camouflage against leaf litter and tannin-stained substrates. Females and non-breeding males are plainer, lacking the vivid distal red of the fins, which are the male's primary display signal. Sexual dimorphism is therefore centred on fin coloration and pattern intensity rather than on body size or shape.
Habitat
In the wild, Parosphromenus opallios inhabits slow-moving to near-still blackwater streams draining peat-swamp forest in Central Kalimantan, Borneo. These are among the most acidic freshwater habitats known: Seriously Fish records pH values around 3 for the type-locality drainage, with temperatures in the low-to-mid 20s Celsius (approximately 72 °F recorded at the site). The water is characteristically very soft, extremely low in dissolved minerals, and heavily stained with humic and fulvic acids leached from partially decomposed peat. Dissolved oxygen is low, a condition the species tolerates through its labyrinthine accessory breathing organ, which allows it to breathe atmospheric air at the surface.
The species is endemic to the Central Kalimantan province of Indonesian Borneo, known definitively from the Arut River basin near Pangkalanbun (approximately 2°04'S, 111°36'E). Within that habitat it occupies sheltered microhabitats among aquatic leaf litter, submerged root masses, fallen branches, and overhanging riparian vegetation, where the current is minimal. These structural refuges serve both as foraging sites and, critically, as potential cave-spawning locations for breeding pairs.
Feeding
Parosphromenus opallios is a micropredator in the wild, feeding on the tiny invertebrate fauna — insect larvae, microcrustaceans, and similar organisms — that inhabit the leaf-litter and detritus of peat-swamp streams. The scale of available prey in these blackwater habitats is constrained by the extreme acidity, so natural prey items are small and often sparse, explaining why the fish itself remains tiny.
In captivity the species accepts live and frozen small foods readily: newly hatched Artemia (brine shrimp) nauplii, Daphnia, and small mosquito larvae (Culex wrigglers) are the standard staples recommended for this genus. Dry or prepared foods are accepted by some individuals but should not form the dietary mainstay; the fish's small mouth limits pellet size, and condition and reproductive willingness are strongly tied to regular provision of live or high-quality frozen invertebrates.
Mating
Parosphromenus opallios is a cave-spawning labyrinth fish. Males establish and defend a small cave territory — a crevice under bark, a rolled leaf, a cave formed by root masses or stone overhangs — and attract females with fin-spreading displays that foreground the iridescent red of the unpaired fins. The contrast between the dark body and the luminous distal fin colour is the primary courtship signal, and males intensify this display at the entrance to their chosen cave site.
Pair bonds in this genus are described as temporary: a male attracts a female to his cave, spawning occurs, and the female departs while the male takes over sole parental duty. Females show mate choice based on display intensity and cave quality; competition among males is largely visual rather than violent, though territorial defence of the cave site occurs. Because the fish inhabit low-density populations in structurally complex microhabitats, encounters between individuals in the wild are likely infrequent, and the high visual impact of breeding-condition males appears adapted to signalling across the leaf-litter landscape.
Breeding
Spawning takes place inside the male's cave. The pair enter together, typically orienting along the ceiling of the enclosure, where the male wraps the female in an embrace (as in all Osphronemidae spawning) and small clutches of eggs are deposited and fertilised. Eggs may be placed individually or in a small cluster on the cave ceiling or inner walls; the male tends them by fanning and guarding. The EAZA Best Practice Guidelines (2024) confirm this cave-spawning mode and note that the male guards the nest until the larvae develop and the free-swimming fry leave the cave, after which parental care ceases.
In captivity, breeding requires replication of the species' extreme water chemistry: very soft, very acidic water (pH 4–5 is a practical target; some breeders work closer to 5–6) at temperatures around 75–79 °F. Cave structures are essential — short sections of PVC pipe, ceramic caves, curved pieces of cork bark, or crevices under slate are all used. A dimly lit tank with a substantial leaf-litter layer on the substrate, dense planting or floating cover, and minimal water movement mirrors the sheltered peat-swamp microhabitat and reduces the stress that inhibits spawning in this sensitive genus. Fry emerge from the cave as free swimmers and, being very small, require infusoria or commercially prepared micro-fry foods as first foods before advancing to Artemia nauplii.
In the aquarium
Parosphromenus opallios is an advanced-level aquarium fish, appropriate for specialists willing to maintain the unusually soft and acidic water conditions the species demands. A small, dedicated species tank or a carefully assembled community of similar blackwater species (other Parosphromenus, small Sundanese rasboras, Boraras species) is the typical approach; the fish is easily outcompeted, stressed, or injured by larger or more boisterous tankmates. A minimum footprint of around 40 × 8 in for a pair is cited in the literature, though a somewhat larger vessel provides more stable chemistry and more room for a male to establish a cave territory without constant stress.
Water quality is the central challenge. Blackwater conditions — pH 4–6, very low hardness, temperature around 72–79 °F — must be achieved and maintained through the use of RO or rain water remineralised to very low conductivity, together with botanical additions (Indian almond leaves, alder cones, or peat filtration) that acidify and tint the water naturally. Standard tap water is unsuitable unless it is exceptionally soft. Filtration should be gentle — a sponge filter at low flow — since the species is adapted to near-still water and strong currents are stressful. Dense cover, dim lighting, and minimal disturbance are key; a well-set-up blackwater tank with abundant leaf litter and cave structures will bring out both natural behaviour and male colour in a way that a bare or bright tank never will.
Conservation
Parosphromenus opallios is assessed as Endangered (EN) on the IUCN Red List, with the assessment dated 14 January 2019. The threat is straightforward and shared by most of its genus: the peat-swamp forests of Kalimantan are being drained, burned, and converted to plantation agriculture at a rate that has destroyed the majority of the lowland peat ecosystem over the past few decades. Because the species is endemic to Central Kalimantan with no known populations outside the Arut River area, any large-scale habitat loss in that basin directly reduces the global population.
The genus Parosphromenus as a whole is the focus of conservation concern within the aquarium hobby: the Parosphromenus Project (parosphromenus-project.org), a dedicated hobbyist conservation initiative, coordinates captive-breeding efforts for multiple species in the genus and maintains breeding groups across participating aquarists. Responsible acquisition of tank-bred rather than wild-caught specimens — verified by provenance from known breeders or hobbyist networks rather than import chain — is the primary lever available to the individual aquarist. The EAZA Best Practice Guidelines published in 2024 formalise husbandry standards for the genus in zoological collections, reflecting recognition of conservation value beyond the specialist hobby.