Taxonomy & naming
Nanosphronemus gemma was described in 2026 by Ralf Britz, Heok Hui Tan and Lukas Rüber in the Raffles Bulletin of Zoology (volume 74, p. 607). The paper erects a new genus for it. The describers found that it differs from every other anabantoid genus in its dorsal- and anal-fin ray counts, its colour pattern and several skeletal characters. Their molecular phylogeny places Nanosphronemus as the sister group of Trichopsis, the croaking gouramis, so the atlas groups it with them.
The Catalog of Fishes lists it as valid in Osphronemidae, subfamily Trichogastrinae. The holotype is ZRC 70244 and the paratypes are ZRC 70070 (14 specimens). The genus name is masculine. Before the description the fish was sold as "Microsphromenus sp. Vietnam" or the "Pho Xua gourami". Microsphromenus is not a valid genus, so that label was only a placeholder. The suggested common name is Jewel gourami.
Morphology
This is one of the smallest anabantoids known. The describers illustrate live fish of about 0.5 in standard length. The dorsal fin has XIX–XXI spines followed by 2–3 soft rays, and the anal fin has XX–XXIV spines followed by 4–5 soft rays. No other anabantoid genus has this combination of counts.
The colour pattern is also unique to the genus. A mid-lateral stripe runs along the body. A putative female lacks the red stripe in the dorsal half of the body. A putative male has a red stripe above the mid-lateral stripe and darker pigment below it. In breeding colour the male is darker overall, including the fins. His red stripe is bright, and the scales in the mid-lateral stripe and below it on the head and body are highly iridescent.
Habitat
The fish comes from wetlands in the vicinity of Huế, central Vietnam. The describers deliberately do not disclose the exact locality, so the atlas gives none. The type locality is recorded only as an undisclosed area near Huế.
No water measurements from the collecting site are published. The atlas therefore gives no temperature, pH or hardness range for the wild habitat.
Feeding
Wild diet has not been documented. The seller Wet Spot Tropical Fish describes the fish as an insectivore. It advises offering small live and frozen foods such as daphnia, baby brine shrimp and microworms. Wet Spot also warns against overfeeding.
Mating
The describers photographed a mature female carrying eggs. They also show a male in breeding colour in an oblique courtship position. Details of the courtship beyond these photographs are not documented.
The sexes are hard to tell apart. Wet Spot calls the fish nearly impossible to sex and recommends keeping a group so that pairs can form. The colour differences noted by the describers apply to putative males and females.
Breeding
Nanosphronemus gemma is a bubble-nester. The describers' figures show a male approaching a bubble nest under a plant leaf. Another figure shows a male adding air bubbles and embedding eggs into the nest.
Clutch size, hatching time and fry care are not documented in the description. The atlas gives no figures for them.
In the aquarium
All care advice here comes from the seller Wet Spot Tropical Fish, not from long-term keeping records. Wet Spot lists adults at about 0.5–0.7 in. It gives a minimum of 5 US gallons for a pair, and more for a group.
Wet Spot describes the fish as territorial. Sparring between members of the same species is common. Serious injury is avoided with ample space and cover. Feed small live and frozen foods in small amounts. Wet Spot sells wild-caught fish under the trade name Pho Xua Gourami.
Conservation
The species has not been assessed by the IUCN, and the atlas knows of no assessment. Its range is known only as wetlands near Huế, and the describers withhold the exact site. The fish entered the trade before it was described, and its wild population status is not documented.