Taxonomy & naming
The German-American ichthyologist Carl H. Eigenmann named the species in 1912, in his monograph 'The freshwater fishes of British Guiana,' as Aequidens potaroënsis (Memoirs of the Carnegie Museum 5(1):490, plate 66). The type locality is Amatuk, a cataract on the Potaro River in the Essequibo drainage of Guyana. Its holotype is held at the Field Museum of Natural History (FMNH 53892, originally Carnegie Museum CM 2407), with a long series of paratypes spread among the Carnegie/Field, British Museum (Natural History), California Academy of Sciences, Vienna, Berlin, Amsterdam and Smithsonian collections. The Catalog of Fishes and FishBase both keep the original combination, listing the fish as a valid Aequidens (subfamily Cichlinae) after Kullander's 2003 checklist. Aequidens, though, has long been recognised as a non-monophyletic grab-bag of mid-sized 'acaras,' and this species is widely regarded as misplaced in it. Describing the related Krobia petitella, Steele, Liverpool and López-Fernández (2013) noted that 'Aequidens' potaroensis — like the Surinamese 'Aequidens' paloemeuensis — shares several diagnostic traits with Krobia: three anal-fin spines, lower vertebral and E1 scale counts than true Aequidens, cycloid prepelvic scales, the facial stripes, and a suborbital stripe kept into adulthood. On that basis the Cichlid Room Companion and others now treat it as Krobia potaroensis (Eigenmann, 1912), in tribe Cichlasomatini. One trait still holds it slightly apart from typical Krobia: Kullander (2012) found its predorsal scales in three rows (triserial), as in other Aequidens, rather than the single row of 'Aequidens' paloemeuensis. The generic placement is genuinely in flux, so this profile keeps the original name Aequidens potaroensis while noting that Krobia potaroensis is the increasingly favoured combination.
Morphology
Aequidens potaroensis is a small cichlid, reaching roughly 4 in in standard length (Kullander 2003) — modest even by acara standards. It has the laterally compressed, moderately deep-bodied build typical of the Cichlasomatini, and the colour pattern that ties it to Krobia and the smaller 'acaras': a brownish to greyish ground, faint vertical cross-bands on the flanks, a dark bar that runs from the eye down across the cheek to the edge of the preopercle, and — diagnostic for this group — a suborbital stripe beneath the eye that persists into adulthood. Boeseman (1954) recorded the fin formula of one Surinam-area specimen as roughly D.XV (about four to ten spines plus ten soft rays, though the spinous portion of that fish was deformed) and A.III,9; those three anal spines are among the characters that ally it with Krobia rather than true Aequidens. No detailed account of sexual dimorphism has been published for this species. By analogy with its relatives, males probably grow somewhat larger and develop more extended dorsal- and anal-fin tips, but that is inference rather than documented fact, and dimorphism here should be treated as unconfirmed.
Habitat
The species is endemic to the Essequibo River basin of Guyana, on the eastern margin of the Guiana Shield. Eigenmann's types came from Amatuk on the Potaro, and later survey work (summarised by Steele et al. 2013) records it from both lowland and upland reaches of the Mazaruni and Potaro Rivers as well as the Essequibo itself. FishBase describes its habitat as fast-flowing 'blackwater' streams (after Stawikowski & Werner 1998) — tea-coloured, soft, acidic water stained by tannins leached from forest leaf litter, a water type widespread across the Guiana Shield. The Potaro is the river of Kaieteur Falls, where the water drops some 741 ft off the Pakaraima escarpment before threading a long gorge to join the Essequibo; the surrounding ecoregion mixes forested uplands full of rapids and cataracts with a low coastal plain of seasonally flooded rivers, lakes and swamps. No in-situ temperature, pH or conductivity readings specific to this fish have been published, but field chemistry on its actual type river is on record: a University of Guyana survey of the Potaro near Kaieteur National Park (Davis, Oyedotun & Simmons 2020), sampling above and below the falls, found mean pH spanning 3.4 to 7.7 and conductivity from essentially nil up to about 25 microsiemens per centimetre — extremely soft, ion-poor, often strongly acidic water. Temperatures in these warm lowland-to-upland streams sit broadly in the mid-20s Celsius, with a Potaro biotope record of roughly 75–82 °F across seasons. These figures are basin-typical rather than measured on potaroensis itself, but they bracket the conditions it lives in. The Essequibo ecoregion is rich in Guiana-Shield endemics — 58 endemic fish species and several endemic genera — and this fish belongs to that restricted-range fauna.
Feeding
No gut-content or feeding study has been published for Aequidens potaroensis. FishBase assigns it an estimated trophic level of about 3.2 — a figure modelled from its size and from the diets of close relatives, not measured directly — which would make it a small, carnivore-leaning omnivore. That fits the general ecology of the Krobia–'acara' group, whose members are typically opportunistic micropredators and foragers: they take aquatic insect larvae and other small benthic invertebrates, rounded out with detritus and plant material picked from the substrate and leaf litter. Absent direct data, the most defensible statement is simply that it is a small generalist invertivore-omnivore of Guiana-Shield streams; anything more specific about its diet would be unsupported.
Mating
The pre-spawning behaviour of this species has not been described in the wild, and there is little aquarium literature to fall back on. What can be said comes by extension from its biology as a substrate-spawning cichlid and from the better-known members of the Krobia and 'acara' assemblage. Such fish generally form monogamous pairs that stake out and jointly defend a small territory around a chosen spawning site, with courtship built on mutual displays, lateral posturing, fin-spreading and substrate cleaning before the eggs are laid. Whether potaroensis pairs off from a group, how aggressive it is toward its own kind, and what its courtship looks like in detail are all undocumented. This is one of the sections where the honest answer is that the behaviour simply has not been studied.
Breeding
Aequidens potaroensis is a biparental substrate spawner. The one concrete observation in the literature, reported through FishBase from Stawikowski & Werner (1998), is that it spawns on hard substrate and that both parents defend the eggs and larvae — the classic substrate-brooding mode of the cichlasomatine cichlids. In that pattern the female tends and fans an adhesive clutch laid on a cleaned rock or similar surface, both partners guard the nest, and once the fry are free-swimming the pair shepherds them together. Clutch size, egg and larval development times, the precise nest type, and any seasonal or chemical spawning triggers have not been documented for this species, and no detailed aquarium breeding report exists — the fish is only occasionally seen in the hobby. Beyond 'biparental, hard-substrate, both parents guard,' the reproductive details remain unrecorded.
In the aquarium
Aequidens potaroensis is rarely seen in the hobby and essentially absent from commercial export — it has no trade name, turns up only in sporadic specialist shipments from Guyana, and carries no aquarium literature of its own. The guidance below is drawn from its biology, its confirmed wild-water chemistry, and the well-kept relatives it most resembles (Krobia guianensis and Krobia itanyi).
Tank: a pair suits a 35.5–39.5 in aquarium (roughly 25–30 US gal net); a lone fish needs only a 23.5–31.5 in tank, but the group's breeding biology makes a matched pair the natural unit, and the larger footprint helps buffer aggression around spawning. Coming from shallow, fast-flowing streams, it prefers moderate current over a still, heavily planted tank — a canister return aimed along the surface, or a powerhead on low.
Water: match the wild profile — soft (1–4 °dH, certainly below 8 °dH), slightly to moderately acidic (pH 5.5–7.0; the pH 3–4 extremes recorded in the Potaro reflect peak blackwater conditions and need not be reproduced), and warm at 75–81 °F. It likely tolerates neutral, moderately soft tap water as adults, like Krobia guianensis and Krobia itanyi, but avoid very hard alkaline water and keep conductivity low.
Décor: fine-to-medium sand for its digging, flat stones and smooth pebbles as spawning sites, driftwood for refuge and tannins, and a layer of dried catappa or other broadleaf litter to soften the water and mimic the forest-floor streambed. Floating plants that shade the surface are welcome.
Tankmates: a breeding pair guards its patch fiercely and can injure or kill small fish, so choose carefully. In a large tank, peaceful Guiana-Shield tetras such as Hemigrammus and Moenkhausia make naturalistic dithers, and soft-water Corydoras suit the non-breeding periods; avoid other similar-sized substrate-spawning cichlids except in very large quarters. Away from spawning it is only moderately assertive.
Feeding is easy: it takes micro-pellets, flake, and frozen or live bloodworm, Daphnia and brine shrimp without hesitation — feed twice daily and vary the menu.
Difficulty is low to moderate. Once the water is right the fish is hardy, and for a seasoned South American cichlid keeper it is straightforward; the real obstacles are sourcing a pair and supplying soft, acidic water, not any fragility. It is not an ideal first cichlid, mainly because of its rarity. Aquarium experience is too thin to pin down species-specific mistakes, but by analogy with Krobia guianensis the likeliest error is keeping it in hard, high-pH water long-term, which may suppress spawning and shorten its life.
Conservation
Aequidens potaroensis has not been evaluated by the IUCN Red List (status Not Evaluated), and it carries no CITES or CMS listing. As a narrow-range endemic of the Essequibo system it has a naturally limited distribution, but there is no evidence of any specific threat to it and no population data on which to base a status. The wider Essequibo ecoregion remains relatively intact and is described as only fairly to poorly explored, especially in its headwaters and small streams. The broad pressures on Guiana-Shield freshwaters — small-scale and increasingly mechanised gold mining, with its sedimentation and mercury contamination, forest clearance, and the prospect of further hydrological change on rivers like the Potaro and Mazaruni — are the realistic long-term concerns for any fish of these waters, but none has been documented as affecting this species in particular. Given how little is known of its biology, abundance and exact range, it is best described as a data-poor endemic rather than a fish of known conservation concern.