Taxonomy & naming
Parosphromenus ornaticauda was described by Maurice Kottelat in 1991. The type locality is the area downstream of the Kapuas River near Anjungan and Mandor in Kalimantan Barat (West Kalimantan), Indonesian Borneo. The valid name as recognised by Eschmeyer's Catalog of Fishes is Parosphromenus ornaticauda Kottelat, 1991.
The species belongs to the parvulus species group within Parosphromenus, a genus of small labyrinth fishes in the family Osphronemidae. All members of the genus are sometimes called licorice gouramis in the hobby, a name that loosely reflects their small size, cryptic habits, and often striped or dark patterning. Parosphromenus is restricted to the Sundaland region of Southeast Asia — the Malay Peninsula, Sumatra, and Borneo — and its species are among the most soft-water-dependent labyrinth fishes known.
Morphology
Parosphromenus ornaticauda is a very small fish, reaching approximately 0.5 in standard length. Like other members of the genus it is slender and elongate compared with the deep-bodied gouramis of commerce, with relatively large fins and subdued background coloration overlaid by iridescent markings.
The specific epithet ornaticauda means 'ornate tail', referring to the patterned caudal fin that distinguishes this species — a characteristic that, combined with fin coloration and pattern, is the primary tool for separating species within the cryptically similar parvulus group. Males in breeding condition display more vivid colours; females are plainer and typically rounder in the belly when gravid. The suprabranchial labyrinth organ is present as in all anabantoids, allowing the fish to breathe atmospheric air directly.
Habitat
In the wild, Parosphromenus ornaticauda inhabits peat swamp forest streams and pools in the lowlands of West Kalimantan. These are blackwater environments of extraordinary chemical extremity: pH values of 4.1–4.5 have been recorded at the type locality, conductivity below 40 µS/cm, and temperatures around 81–82 °F. The water is stained deep brown with dissolved humic and fulvic acids leached from peat, and dissolved oxygen may be low in still pockets, which the labyrinth organ compensates for.
The fish shelters among submerged leaf litter, tree roots, and marginal vegetation in the shallows of peat swamp pools and small streams. The habitat is characterised by near-total absence of dissolved minerals, negligible buffering capacity, and a rich organic substrate — conditions that generalist aquarium fish cannot survive but that Parosphromenus species have evolved to exploit. The Anjungan–Mandor area lies in the Kapuas River drainage of West Kalimantan.
Feeding
Parosphromenus ornaticauda is a micropredator in the wild, feeding on very small invertebrates: tiny crustaceans, insect larvae, worms, and other zooplankton-sized organisms found among leaf litter and at the water surface in its peat swamp habitat.
In the aquarium the species accepts small live and frozen foods readily: newly hatched Artemia (brine shrimp nauplii), Daphnia, mosquito larvae, and similarly sized prey. Prepared foods are taken less reliably; because of the fish's minute size, only very small particles are manageable. A diet of varied live and frozen invertebrates is strongly recommended for conditioning breeding pairs and for rearing fry.
Mating
Parosphromenus ornaticauda is a cave spawner. In the wild it uses small crevices, the undersides of leaves, root tangles, or similar sheltered micro-habitats as spawning sites. In captivity, small PVC tubes, coconut shell halves, or folded leaf litter serve as artificial caves.
The male establishes and defends a spawning cave, displaying to females at the entrance. Courtship involves fin-spreading and colour intensification; the male's ornate caudal fin plays a role in species recognition and mate assessment within the genus. Displaying males are most vivid under dim light in a heavily furnished, darkened tank. As with other Parosphromenus, pair compatibility can vary and it is advisable to allow fish to select partners, or to use an established compatible pair, rather than forcing an arbitrary male–female pairing.
Breeding
Spawning takes place inside the cave. The female enters, the pair embrace, and small clutches of eggs — typically 10–20 per spawning event according to hobbyist records — are deposited on the ceiling or interior surface of the cave, often accompanied by a small cluster of bubbles that help suspend them. The male guards the eggs after spawning.
Breeding success requires very soft, highly acidic water closely mimicking the natural blackwater: pH 3.0–4.0 and high humic acid content (achieved with peat filtration or blackwater extract) are reported as necessary. At these extremes bacterial infection of eggs is reduced and hatching rates improve. Eggs are sensitive to disturbance — they may be relocated or eaten if conditions are suboptimal or if the fish feel threatened. Fry are tiny and require infusoria or the smallest Artemia nauplii as first foods. Water quality stability is critical; even small parameter shifts can cause losses among the fry.
In the aquarium
Parosphromenus ornaticauda is regarded as one of the more demanding licorice gouramis to keep and breed successfully, largely because of its uncompromising water chemistry requirements. A small, well-established blackwater aquarium with heavily tannin-stained, peat-filtered water, pH in the range 3.5–4.5, negligible hardness, and stable temperature around 79–82 °F is the target environment. Filtration should be gentle — a small sponge filter or air-driven system — to avoid disturbing the fine leaf litter substrate and to keep surface agitation minimal while still circulating humic-rich water through peat.
The tank should be dimly lit, heavily planted or furnished with dried leaf litter, driftwood, and cave structures to provide refuges and spawning sites. Because the fish is so small and surface-access to breathe atmospheric air is essential for a labyrinth fish, the tank must be covered and the air gap kept warm and humid. The species is best kept as a dedicated species tank or with only the most peaceful, similarly sized blackwater companions. It should not be kept with larger, boisterous, or hardwater species. Sourcing captive-bred stock from specialist licorice gourami keepers is strongly preferable to wild-caught fish.
Conservation
Parosphromenus ornaticauda is assessed as Critically Endangered (CR) on the IUCN Red List (assessment 2019, e.T91315657A91315674). The assessment reflects the species' extremely restricted range — it is known from peat swamp areas near Anjungan in Kalimantan Barat — combined with severe and ongoing loss of that habitat. Lowland peat swamp forests in Borneo have been converted to oil-palm plantations at a large scale; the original habitat at and near the type locality has been extensively destroyed.
Additional pressures noted in the IUCN assessment include aquarium trade collection and continued habitat degradation from drainage, agricultural pollution, and fire. With so little habitat remaining and no indication that decline has slowed, the species faces a genuine extinction risk in the wild within a generation if habitat loss continues at current rates.
The Parosphromenus Project, an international network of hobbyists dedicated to maintaining and breeding endangered Parosphromenus species, actively works with P. ornaticauda and publishes husbandry notes for the species. Captive populations held by project participants represent an important buffer against wild extinction. Aquarists interested in the species are encouraged to connect with the project and to source only captive-bred fish.