Taxonomy & naming
The Swedish ichthyologist Sven O. Kullander described Aequidens patricki in 1984, in the French journal Revue française d'Aquariologie et Herpétologie (volume 11, number 1, pages 1-6). Its title pinned down the range at a stroke: 'Une nouvelle espèce d'Aequidens des bassins des Rios Aguaytia et Pachitea au Pérou.' The holotype (Muséum d'histoire naturelle de Genève, MHNG 2163.93) came from a small brook reduced to a drying pool — a tributary of the Río Aguaytía at roughly 9°02'S, 75°31'W in Coronel Portillo Province, Peru — with paratypes spread across European museums in Geneva, Brussels, Stockholm, Bonn and Hamburg. Kullander redescribed the species two years later, in his 1986 monograph on the cichlids of the Amazon drainage of Peru, and every checklist since has treated it as valid, including the standard South American catalogue (Reis et al. 2003). The genus Aequidens was long a 'wastebasket' for medium-bodied Neotropical cichlids, and it has since been dismantled piece by piece: the Andean-slope blue acaras moved to Andinoacara, and other groups split away. What remains is a tighter assemblage of 'true acaras' to which Aequidens patricki belongs, within the subfamily Cichlinae among the lowland and foothill acaras of the western Amazon.
Morphology
Patrick's acara is a compact, laterally compressed fish of about 4.5 in. FishBase gives a maximum of 4.5 in standard length, while hobby and reference sources put fully grown males at around 4.5–5 in and females a little smaller. The build is the familiar acara silhouette: moderately deep and ovoid, with a gently sloping forehead and a terminal mouth. What lifts it out of the run of drab Aequidens is the colour — a warm brassy to greyish-olive ground scattered with iridescent blue-green spangling across the flanks, cheeks and unpaired fins, brightening when a fish is in condition. The clearest field mark is a dark blotch set roughly mid-body on each flank, the lateral spot common to the genus, often over faint vertical bars that come and go with mood. Sexual dimorphism is modest and size-based: dominant males grow larger and heavier-headed, ripe females turn rounder-bellied, and as in most acaras the difference sharpens at breeding time rather than showing year-round. There is no dramatic nuptial dress here, but courting fish darken their markings and heighten the blue.
Habitat
The species is endemic to a very restricted part of the western Amazon: the Aguaytía and Pachitea river systems, both draining the Andean foothills into the Ucayali within Peru's stretch of the Amazon basin. Eschmeyer's Catalog of Fishes and the IUCN treat it as a Peruvian endemic of these two drainages. A few aquarium aggregators also list Brazil and Colombia, but the primary taxonomic literature does not support those wider claims, which are best read as range creep. Recorded elevations run from about 623–1969 ft, placing the fish in foothill and upper-lowland country rather than the deep floodplain. Field accounts describe slow to moderate streams of clear and white water over sand, gravel or rock; the type itself was netted from a brook that had shrunk to a standing pool, a reminder that these are seasonal, sometimes intermittent waters. The chemistry is soft and acidic to circumneutral — FishBase lists a lower pH bound near 5.8 and hardness of about 3-19 dH, while biotope sources cite roughly pH 5-6 and very soft water (around 1-4 dH) in the most blackwater-influenced reaches. Temperatures sit on the cool side for an Amazonian cichlid, clustering around 70–77 °F in FishBase and field data, though the fish takes to warmer aquarium conditions readily.
Feeding
No detailed wild gut-content study of Aequidens patricki has been published, so its diet is inferred from its body plan, its genus and the handful of keepers who have worked with it. FishBase places it at a trophic level near 3.7 — a small, generalist predator rather than a herbivore or detritus-sifter. In practice it behaves as a benthic micro-carnivore and opportunistic omnivore: a stream acara that picks invertebrates, insect larvae, small crustaceans and other animal prey from the substrate and water column, rounding the menu out with whatever plant and detrital matter it meets. Aquarium accounts fit that picture. The fish takes most chunky foods readily, from prepared pellets and flake to frozen and live invertebrates, and it is large and predatory enough to treat small surface fishes such as hatchetfishes as prey. In its foothill-stream community it works the bottom as a mid-sized invertebrate feeder.
Mating
Patrick's acara lives in small social groups built around a hierarchy, and like most acaras it grows markedly territorial as it matures — most of all when it comes into breeding condition. Out of season it is comparatively peaceful for the genus and can be kept in roomy community settings, but pairs form by mutual selection from a group, then claim and hold a defended patch. The genus carries the 'wife-beater' reputation common to substrate-spawning cichlids: in confined quarters an unready or mismatched female can be harassed or even killed by an over-eager male. Experienced keepers manage this exactly as they would for related acaras — a large, well-structured tank with broken sightlines and hiding places, or conditioning the female separately and reintroducing the male behind a divider so the bond can settle before the two are in full contact. Courtship then follows the standard acara pattern: mutual displays, jaw-locking trials of strength, and the joint cleaning of a spawning site.
Breeding
No published wild breeding study exists for Aequidens patricki, but its mode is well constrained by the genus: the true acaras are biparental, open substrate spawners, not cave brooders or mouthbrooders. The pair cleans a flat surface — a stone, a leaf, a piece of wood, or the aquarium glass — and the female lays an adhesive clutch in the open, which both parents then tend, the female fanning and mouthing the eggs while the male patrols the territory and drives off intruders. Once the eggs hatch, the parents typically move the wrigglers to one or more pits dug in the substrate, then guard the free-swimming fry as a school, shepherding them and herding strays back — the classic Neotropical substrate-spawner pattern of extended biparental care. Reported aquarium practice points to a modest temperature drop and then a slow rise to 75–77 °F, backed by frequent small changes of cooler, very soft water and a rich live and frozen diet, as the way to bring a conditioned female round. The species is bred in Europe, though wild-caught fish still make up most of the little that reaches the trade, and specific clutch counts and fry-development figures for patricki itself are not reliably documented.
In the aquarium
Aequidens patricki reaches the trade only rarely, and mostly as wild-caught fish from the Peruvian Ucayali drainage, with the occasional European captive-bred specimen; expect to hunt for months rather than find it at a shop.
Tank: a pair or small group needs a well-filtered tank of at least 47 in (roughly 55 US gal), favouring length over height for these bottom-patrolling fish. Fine sand or fine gravel suits their digging; add flat stones and smooth wood as open spawning sites, and plant the back and sides with driftwood roots to break sightlines and absorb aggression. Gentle to moderate, well-oxygenated flow is plenty.
Water: keep to the fish's foothill-stream origins — 70–77 °F (the lower 70–73 °F end is probably healthier long-term) and soft, slightly acidic water at pH 5.5-6.5, hardness under about 10 dH. Harder, alkaline tap water will not kill it outright but tends to suppress breeding and dull colour, so peat filtration or a piece of driftwood is a practical fix.
Diet: undemanding. It accepts good cichlid pellets or sticks but does best on a varied menu of frozen bloodworms, Artemia, Mysis and occasional live invertebrates such as earthworms.
Tankmates: once fully grown it will eat small surface fish — pencilfish, hatchetfish, small tetras — so choose robust mid- and bottom-dwellers of similar size, such as larger characins, callichthyid catfish or comparable slow-water Peruvian species.
Common mistakes: the real difficulty is pair compatibility, since a male can harass an unready female relentlessly, sometimes fatally. Grow on a group of youngsters and let a pair form before removing the spares, or condition the female apart and introduce her behind a divider; a settled pair is manageable, and it is the early stages or any forced pairing that carry the risk.
Conservation
The IUCN Red List assesses Aequidens patricki as Least Concern (assessed 24 April 2014, published 2016; assessors Ortega Torres, Correa & Chuctaya). The reasoning is straightforward: the fish is reported frequent within its range, its population is presumed stable, and no major threats were identified — though the assessment is flagged as needing an update, and notes that the species was not recorded from any protected area. It carries no CITES listing. That clean bill of health deserves two cautions. First, this is a genuinely narrow-range endemic, confined to two foothill drainages of the upper Ucayali; small ranges leave little buffer against localized habitat change, and the type locality itself was a brook in the act of drying out — a measure of how seasonal and vulnerable these headwater streams can be. Second, the wider Peruvian Amazon faces mounting pressure from deforestation, road-building, agriculture and small-scale gold mining, with its attendant mercury and siltation; a Cichlids Yearbook field account of a 1990s collecting trip to the type locality specifically notes the Aguaytía valley as an area of significant coca cultivation, one more concrete land-use pressure on this narrow-range fish's home drainage alongside the region's broader deforestation and agricultural conversion. None of these is documented as a present danger to this cichlid, but they are the backdrop against which a range-restricted foothill species should be watched.