Taxonomy & naming
The uaru was described by the Austrian ichthyologist Johann Jakob Heckel in 1840, in the second volume of the Annalen des Wiener Museums der Naturgeschichte, on a specimen collected in the Rio Negro of Brazil. Heckel coined both the genus Uaru and the species amphiacanthoides in the same work, making Uaru amphiacanthoides the type species of its genus; the unique holotype (NMW 24808) remains in the Naturhistorisches Museum in Vienna, where Heckel curated the fish collection. The name has stood essentially unchallenged: Eschmeyer's Catalog of Fishes lists it as valid with no junior synonyms, and Sven Kullander retained it without comment in his 2003 checklist of South American cichlids. Uaru is a small genus — only one congener is recognized, Uaru fernandezyepezi Stawikowski, 1989, from the Rio Atabapo on the Venezuela–Colombia frontier. Within the family the genus belongs to subfamily Cichlinae and the heroine cichlids (the Heros group of the tribe Heroini), a Neotropical assemblage of deep-bodied, often laterally compressed fishes that includes Heros (the severums), Hypselecara, Mesonauta, Pterophyllum (the angelfishes) and — most tellingly — Symphysodon, the discus. Uaru's morphological and behavioural similarities to discus are close enough that the two are frequently discussed as near relatives, and the uaru's old hobby nickname, the 'poor man's discus,' captures both the resemblance and the difference in price.
Morphology
Uaru amphiacanthoides is among the larger heroine cichlids, reaching about 10 in standard length per Kullander's checklist and roughly 12 in in total length in the largest wild fish; aquarium references commonly cite 10–12 in. The body is tall, strongly compressed and almost circular in outline — the disc-like profile shared with discus and angelfish — set on a blunt, steep-faced head. Adults are a muted greyish-tan to slate or chocolate brown, and the defining mark is a single large, dark, roughly triangular or wedge-shaped blotch on the rear half of each flank, the feature behind the 'triangle cichlid' name; a dark bar also typically runs through the eye. Few cichlids transform so dramatically with age. Newly imported keepers in the early twentieth century were repeatedly fooled into thinking the life stages were different species: tiny juveniles are boldly mottled and spangled with cream-white spots and blotches on a dark ground, a cryptic livery that dissolves into the plain wedge-marked adult coloration as the fish matures and the body deepens. Sexual dimorphism is weak and unreliable — there is no consistent external way to sex the fish. Females tend to stay slightly smaller, look fuller-bodied, and carry shorter pelvic fins, but these tendencies fail often enough on individual fish that the hobby standard is simply to raise a group and let pairs sort themselves out.
Habitat
The species is native to the central Amazon basin of Brazil, extending into adjacent Colombia, with reports from the Guianas region (FishBase treats the Guyanese record as questionable). FishBase circumscribes the range as the Amazon–Solimões drainage from the Rio Japurá downstream to the Rio Tapajós, together with the middle and lower Rio Negro basin. These are warm lowland waters, and the uaru is closely associated with structure: it shelters and forages among submerged branches, leaf litter and the tangled root masses of the flooded forest along river margins. In the blackwater igapó of the Rio Negro it shares this marginal, root-bound sanctuary with the discus Symphysodon, the two disc-shaped grazers tucking into the same undergrowth. The water it lives in is warm, soft and acidic to near-neutral: FishBase records clear-stream populations at around pH 6.8 and gives a tolerance band of pH 5.0–7.0 with temperatures of about 79–82 °F, while the broader hobby experience places it comfortably into the upper twenties and beyond, in keeping with a fish of sun-warmed shallow forest waters. Depth is not separately tabulated for the species, but as an inhabitant of shallow river-edge and flooded-forest habitat it occupies the top few metres of the water column.
Feeding
The uaru is fundamentally a herbivore — an unusual trophic position among the big Amazonian cichlids and one of the clearest differences between it and the carnivorous discus it otherwise resembles. Stomach analyses of wild fish reveal an omnivore that takes a substantial volume of plant matter alongside an assortment of invertebrates, and in their natural habitat the fish are reported feeding on plants and fruit as well as worms, crustaceans, insects and even freshwater sponges; FishBase places the species at a low trophic level of about 2.8 and notes that it 'needs plants as supplement.' In captivity the herbivorous bias is even more pronounced — experiments with captive fish suggest they are primarily herbivorous, and keepers find that planted uaru tanks simply do not last, because the fish strip the vegetation. As a flooded-forest grazer the uaru functions ecologically much like a browsing herbivore of the river-edge, processing soft plant material, fallen fruit and the small invertebrates that come with it.
Mating
Outside the breeding season uaru are sociable, shoaling fish that prefer the company of their own kind, and the standard advice is to keep a group of six or more so that bonded pairs can form naturally — there is no reliable way to hand-pick a male and female, since the sexes are not dependably distinguishable. As a pair forms it withdraws from the group and takes up a territory, and the otherwise notably peaceful disposition of the species sharpens into territorial defence of the chosen spawning site. Both partners share in this defence: uaru are biparental, and the pair bond, courtship and site-guarding are undertaken jointly rather than by one dominant sex. For all that, they remain comparatively gentle for cichlids of their size, and breeding pairs are far less belligerent toward tankmates than many similarly sized Neotropical cichlids — the aggression is focused, territorial and tied to the nest rather than generalized.
Breeding
Uaru amphiacanthoides is a biparental, open substrate spawner. The pair selects and cleans a flat surface — a smooth rock, a piece of bogwood, or a similar hard substrate — and lays its eggs in the open, after which both parents tend the clutch: the eggs and resulting larvae are guarded and fanned by the pair, with FishBase citing care of up to about 200 eggs (hobby spawnings on larger adults often run somewhat higher). The species' most celebrated reproductive trait is shared with its relative the discus: once the fry become free-swimming, they feed by grazing a nutrient-rich mucus secreted over the parents' bodies, darting in to nip particles directly from the adults' flanks in the manner of discus fry. This contact-feeding is genuine parental provisioning — the same mucus-feeding adaptation seen in discus and a handful of other Neotropical cichlids — though uaru fry, unlike young discus, are not strictly dependent on it and can also be reared on conventional foods such as newly hatched brine shrimp and fine prepared diets. Wild-caught adults are reported to spawn most readily in the very soft, acidic water of their home range, whereas captive-bred stock is markedly less demanding about water chemistry.
In the aquarium
The uaru has been in the hobby for well over a century — first imported around 1913 and bred in captivity by 1916 — and remains a rewarding if specialized fish, sometimes sold as the 'poor man's discus' for its disc-shaped body, social grace and demand for warm, clean water. It is a large cichlid, so space is the first requirement: a single pair wants something on the order of a 47 in (4-foot) tank as an absolute minimum, and because the fish do best as a group, a considerably larger aquarium is preferable for the six-plus shoal that lets pairs form naturally. The tank should be set up to suit a shy, structure-loving forest fish — a sand or fine-gravel substrate, generous bogwood and root décor to provide cover and break sightlines, and subdued lighting. Live planting is effectively futile: the uaru is a committed herbivore and will eat most aquatic plants, so keepers either go without or use hardscape and tough or artificial plants.
Water should mirror the warm, soft, gently acidic forest streams of the central Amazon. A temperature in the high twenties is appropriate — sources cite roughly 81–86 °F, with the species favouring the upper end of the range — and a pH around 6.0–7.0 in soft water (up to about 5 °dH) suits it well; FishBase gives a tolerance of pH 5.0–7.0 and dH 5–12. Wild-caught fish in particular do best, and breed most readily, in very soft, acidic water close to their natural parameters, while captive-bred stock is more forgiving. Clean, well-filtered, well-oxygenated water and the higher temperatures these fish prefer are the non-negotiables; any tankmate must also tolerate that warmth.
Diet should be built around vegetable matter — spirulina-based and other herbivore pellets and flakes, blanched greens, and plant material — supplemented with the invertebrate foods the species also takes in the wild (worms, crustaceans, insect larvae). Keepers report the fish are easy to feed and do well on a predominantly herbivorous menu.
Temperament is the uaru's great virtue: it is surprisingly peaceful for so large a cichlid and can be combined with a wide range of companions, including fish small enough that a more predatory cichlid would eat them. Recommended tankmates include angelfish (Pterophyllum scalare and the warmth-loving altum), severums and other Heros, eartheaters such as Geophagus and Satanoperca, and larger, peaceable characins (headstanders, Prochilodus, silver dollars). The chief caveats are simple: avoid overly aggressive cichlids that will bully these gentle, somewhat timid fish, keep them in groups rather than singly, and make sure every tankmate is comfortable in the warm water uaru require.
Conservation
The IUCN Red List assesses Uaru amphiacanthoides as Least Concern (assessment carried out 7 November 2018 by Brazil's Instituto Chico Mendes de Conservação da Biodiversidade, ICMBio, and P. Reis; population trend stable), reflecting its wide distribution through the central Amazon and the absence of any evidence of broad decline. It carries no CITES or CMS listing. The species is genuinely common across its range — in much of the central Amazon it is caught and traded locally as an inexpensive food fish — and it also supports a steady, low-volume ornamental trade, with both wild-caught adults and tank-raised juveniles offered to aquarists. Neither use is flagged as a population-level threat. As with the whole Amazonian fish fauna, the longer-term backdrop is the accelerating pressure on the basin's freshwaters from deforestation, damming, mining and altered flood regimes; none of these is documented as a specific danger to this widespread, adaptable cichlid, but they frame the context in which its status should be read.