Taxonomy & naming
Nanochromis splendens was described in 1976 by the American ichthyologists Tyson R. Roberts and Donald J. Stewart in their landmark ecological and systematic survey of the fishes of the lower Zaïre (now Congo) River rapids, published in the Bulletin of the Museum of Comparative Zoology. The holotype (MCZ 50476) and an extensive paratype series — held at the Museum of Comparative Zoology, the Natural History Museum in London, the Royal Museum for Central Africa at Tervuren, and the Smithsonian — were taken at the lower Congo River near Inga, at roughly 5°27.5'S, 13°36.0'E. It was one of four rheophilic Nanochromis the two authors named in that single paper, alongside Nanochromis consortus, Nanochromis minor, and Nanochromis parilus, and the name has stood unchallenged: it remains valid as the original combination Nanochromis splendens Roberts & Stewart, 1976, with no synonyms. The genus Nanochromis itself was erected by Jacques Pellegrin in 1904 as a replacement name for Boulenger's 1899 Pseudoplesiops (preoccupied by an earlier marine genus), with Nanochromis nudiceps as its type species; it sits in the African cichlid subfamily Pseudocrenilabrinae. The genus has since been pruned — Greenwood split off Limbochromis and Parananochromis in 1987, and Stiassny and Schliewen removed a cluster of species to the new genus Congochromis in 2007 — leaving Nanochromis as a tighter assemblage of Congo-basin dwarfs. The Austrian cichlid specialist Anton Lamboj, who reviewed the West African cichlids and (with Robert Schelly) described a further Nanochromis in 2006, treats splendens among the rheophilic lower-Congo group.
Morphology
This is a small, strikingly slender cichlid — among the most slim-bodied of an already slim genus. FishBase gives a maximum of about 2 in standard length, and in the aquarium the fish tops out around 2 in, the male a touch larger than the female; reports of much larger sizes circulating in the hobby are errors. The body is elongate and low-slung, built for life close to the bottom in moving water. Colour is the species' calling card. The epithet splendens — Latin for splendid or shining — refers, in the describers' own account, to the canary-yellow and orange-red pigments that develop on the gill covers of both sexes at maturity, set over a base that matures into iridescent blue with pastel and emerald reflections. Males carry long, filamentous black pelvic fins, ornately patterned unpaired fins, and vertical carmine bars across the lower half of the caudal fin. Sexual dimorphism is pronounced and, as in several Nanochromis, partly reversed: the female is shorter and rounder-bodied with a conspicuous belly that shines in reflective green scales, and her ovipositor (vent tube) often protrudes even outside spawning condition — a trait keepers use to sex the fish at a glance. Juveniles are nondescript, mostly brown until about 1.5 in, when blue spreads over the body and the red, orange, and yellow fin colours lock in with maturity.
Habitat
Nanochromis splendens is endemic to the lower Congo River in the Democratic Republic of the Congo, where it lives along the main channel among the celebrated rapids between the Malebo (Stanley) Pool and the cataracts at Inga — importers place its range roughly along the shoreline of the middle stretch between Inga and Kiganga. There it is found together with Nanochromis parilus and, downstream, with Nanochromis consortus, a tidy example of how several closely related dwarfs partition this one violent reach of river. The lower Congo rapids are among the most powerful and hydrologically extreme stretches of freshwater on Earth, and the rheophilic Nanochromis are confined to the rapids proper — over sand or bare bedrock — and are absent from the quieter water between them (Schwarzer, Misof, Ifuta & Schliewen 2011). Contrary to the very soft, blackwater habitats of the genus's lacustrine members in Lake Mai Ndombe, the lower-Congo rapids run comparatively mineral-rich and close to neutral. Roberts and Stewart's original collections recorded pH between about 6.5 and 8.5 in fairly soft water, with temperatures around 76–84 °F (76–84 °F) and sampled depths typically of 3–7 ft. These are warm, fast, well-oxygenated waters, and the species should be understood as a current-loving fish of the open river edge rather than a sheltered backwater dwarf.
Feeding
Like all Nanochromis, Nanochromis splendens is an omnivore that forages low in the water column, working the sand and rock of its rapids habitat for small invertebrates, organic particles, and the biofilm and algae that coat hard surfaces in moving water. FishBase places it at a trophic level of about 3.2, the value of a small generalist that takes a mix of animal and plant material. Detailed wild gut-content data are lacking — the type series was gathered en masse by rotenone over large areas, which does not preserve fine habitat or feeding detail — but the genus's biology and aquarium behaviour are consistent: these are bottom-oriented grazers and pickers that need roughage as well as protein. In captivity the species accepts almost anything offered, gorging on live and frozen foods while also grazing constantly on the substrate and surfaces, and keepers stress that vegetable matter belongs in its diet for long-term health. Within its community it occupies the small-invertebrate-and-aufwuchs niche of the rapid margins, sharing that zone with congeners and with other lower-Congo endemics such as Steatocranus and Teleogramma.
Mating
Pair formation in Nanochromis splendens can be dramatic. The fish are normally sociable and parade openly along the front of the tank, but introducing an unbonded male and female frequently triggers a violent settling-in period: keepers and European aquarists alike report bouts of severe aggression lasting up to three weeks, with dominant males charging the female on every appearance until — often quite suddenly — the two begin swimming calmly side by side. Importers note a practical fix used across the genus: keeping a few other West African dwarf cichlids in the tank gives the pair an outward target, so their aggression is directed at neighbours and they cooperate in defending a shared territory rather than turning on each other. Once a bond forms it tends to be close and durable, the pair remaining in constant contact. As courtship builds, the fish dig lightly and the female solicits at a chosen site; the protruding ovipositor and the increasingly vivid, rounded belly signal her readiness. This combination of unpredictable early aggression and strong eventual pair fidelity is the species' defining behavioural quirk and the main reason it is recommended for experienced keepers.
Breeding
Nanochromis splendens is a biparental cave spawner, in keeping with its genus. The pair selects a secluded, shaded site — in aquaria, observed spawns have used overhangs under root wood and rock structures, often just off the edge of the filter's flow, rather than the enclosed pots and coconut caves supplied; the female may also excavate beneath flat rocks. Eggs are laid hidden inside the chosen cave and tended there by the female, while the male patrols and defends a larger perimeter; after the fry are free-swimming he commonly takes over close guarding while she prepares the next clutch. Spawns are modest: documented captive broods have run from about seven fry up to roughly thirty to fifty, smaller than the prolific output of some congeners. The parents exert unusually tight control over the free-swimming young, which (in fast-water fashion) stay low and close to the bottom and rarely rise above the hovering guardian — a behaviour plausibly adaptive for keeping a brood together in current. Care is genuinely biparental and can extend a month or more, though pairs do not always cooperate, with male and female sometimes splitting a brood between separate territories. Eggs and larvae are well hidden and the spawning is so discreet that keepers often only realise it has happened when fry appear. Frequent water changes, high-quality and live foods, good current, and ample cover are the reported triggers.
In the aquarium
Nanochromis splendens reached the dwarf-cichlid hobby only around 2014, when importers including Oliver Lucanus brought the first living pairs out of the Democratic Republic of the Congo, and it remains an uncommon, sought-after fish for experienced keepers rather than a beginner's cichlid. Despite its tiny size it benefits from generous floor space: a tank of around a metre (three feet) or more gives a pair, or better a small group, room to sort out the fraught early aggression that characterises pairing. The single most common way to lose these fish is to confine an unproven male and female together with nowhere for the weaker fish to escape; dense cover — thickets of plants, complex rockwork with multiple exits, even spawning mops — plus a few other robust West African dwarfs to absorb the aggression, is the proven approach. Provide a sand substrate for digging, several dark caves with entrances barely wide enough for the female, driftwood and rockwork, and subdued lighting.
This is a rheophile, so water should be warm and moving: temperatures of about 77–82 °F (the often-cited setup runs near 79 °F / 78 °F) with two filters or returns positioned to create current along the back of the tank. Unlike the genus's blackwater lake species, the lower-Congo rapids forms do not require extreme soft acid water — a pH of roughly 6.0–7.5 in soft to moderately soft, well-oxygenated water suits them, and slightly acidic, moderately soft tap water has worked for breeders. Frequent, generous water changes are repeatedly emphasised; clean, oxygen-rich water mirrors the natural habitat and keeps the fish in breeding condition.
Feeding is easy: a varied diet of sinking pellets and algae wafers for daily fare, supplemented with live and frozen foods — baby brine shrimp, daphnia, mosquito larvae — for conditioning, with vegetable content kept in the rotation since this is an omnivore that needs roughage. Tankmates should be peaceful and not so boisterous as to overwhelm a secretive pair; small surface-and-midwater fish that like moving water (lampeyes have been used successfully) make good dither fish and stay clear of the guarded fry zone. The honest summary: a beautiful, rewarding, and breedable fish that asks for patience through a turbulent pairing process and clean, flowing water thereafter.
Conservation
The IUCN Red List assesses Nanochromis splendens as Vulnerable under criterion D2 (assessed 16 February 2009 by T. Moelants, published 2010), a category that flags a species with a very restricted range potentially threatened by a plausible future event, rather than one in documented decline; the population trend is listed as unknown. The whole of its known range lies within a single, hydrologically extreme reach of the lower Congo in the Democratic Republic of the Congo, which makes it inherently exposed. The threats the assessment identifies are concrete and local: hydropower development and water management — the existing and long-planned dams at Inga sit squarely within its range — together with mining and quarrying in the lower-Congo corridor. Because the rheophilic Nanochromis are tied specifically to the rapids and cannot persist in the slack water between them, large-scale alteration of the river's flow regime is the central concern for the species' long-term survival. It carries no CITES or CMS listing and is harmless to humans; its modest presence in the ornamental trade is not flagged as a population-level threat.