Taxonomy & naming
The Brazilian zoologist Alípio de Miranda Ribeiro described the species in 1918 as Acaropsis rondoni, in the Cichlidae fascicle of the natural-history reports of the Rondon Commission — the Comissão de Linhas Telegráficas Estratégicas de Mato Grosso ao Amazonas. His account was brief, with a single plate. The holotype, MNRJ 1616, is held in the Museu Nacional in Rio de Janeiro, and the type locality is the Rio do Sangue, above Bello falls — a clearwater tributary system of the upper Tapajós basin in Mato Grosso. Acaropsis later proved to be preoccupied, so the fish was transferred to Aequidens, a placement Sven O. Kullander confirmed in the 2003 Checklist of the Freshwater Fishes of South and Central America. It now stands as Aequidens rondoni (Miranda Ribeiro, 1918). Aequidens belongs to the subfamily Cichlinae, tribe Cichlasomatini. Once a catch-all for dozens of medium-sized South American acaras, it has been dismantled piece by piece, its former members reassigned to genera such as Andinoacara, Bujurquina, Laetacara, Cleithracara, Krobia, and Tahuantinsuyoa. One flank-pattern detail separates Aequidens rondoni from its neighbours: the 2023 description of the related Aequidens pirilampo notes that Aequidens rondoni carries effectively nine flank bars, one more than the eight of Aequidens pirilampo. No junior synonyms are recognized beyond the original Acaropsis combination.
Morphology
Aequidens rondoni is a small cichlid. FishBase lists a maximum of 4 in standard length, taken from Kullander's checklist, while the holotype and paratype males measured roughly 3.5–4 in — figures echoed by hobby sources that put typical size at about 3–4 in. The build is a conventional acara's: a moderately deep, laterally compressed body with a rounded head and the continuous dorsal fin typical of the genus. Its colours are subdued rather than showy — a brownish to olive ground crossed by about nine dark vertical flank bars, with the usual acara spot on the midflank. The sexes differ only modestly, as in Aequidens generally: males grow somewhat larger and develop more produced tips to the soft dorsal and anal fins, while ripe females are smaller and fuller-bodied. No account of dimorphism specific to Aequidens rondoni has been published, so those notes describe the genus rather than dedicated study of this fish.
Habitat
The species is endemic to Brazil and, on current records, confined to the upper Tapajós River basin — the Rio do Sangue and the associated Juruena headwaters on and around the Chapada dos Parecis in Mato Grosso. Both the Eschmeyer Catalog of Fishes and FishBase file it under the Amazon basin by way of the upper Tapajós, and GBIF occurrence records cluster in the same corner. The Tapajós-Juruena is a classic clearwater system, draining ancient crystalline shield and sandy beds, and its upper reaches run warm, soft, acidic, and low in dissolved minerals. Hobby observations fit that picture: a benthopelagic fish of acidic, soft, dark or clear water, swimming a metre or more above the sand among rocks and submerged wood. No in-situ water-chemistry dataset has been published for the species itself. The usual aquarium figures — soft, acidic water around 75–82 °F and pH roughly 5 to 6.5 — are extrapolated from the basin and from related acaras rather than measured in this fish's own streams, so they are best read as approximate.
Feeding
No gut-content study of Aequidens rondoni exists. By the standard of its genus it is an opportunistic benthic omnivore: FishBase estimates a trophic level near 3.2, in keeping with a diet of small invertebrates, insect larvae, and detritus taken from and just above the substrate, with small prey added when available. Its larger and far better-studied relative Aequidens tetramerus, the saddle cichlid, feeds in exactly this generalist way, and Aequidens rondoni is presumed to fill a comparable forager's role in the leaf-litter, sand, and woody debris of upper-Tapajós streams. Accounts that cast it as more carnivorous — taking small fish and fry — are plausible for an acara of this size, but they rest on aquarium impression rather than field data.
Mating
Mating has not been recorded in the wild for this species, and no reliable captive-spawning account specific to it has been published; what follows is drawn from the genus. Aequidens are monogamous, biparental cichlids that pair off and defend a territory around a chosen spawning site. Outside breeding they are only moderately aggressive, but a pair coming into condition turns far more territorial, and a bonded pair guards the nest area jointly against all comers, conspecifics included. Pair formation runs through the usual cichlid sequence — mutual display, substrate cleaning, and escalating defence of the site. Any finer claim about the courtship of Aequidens rondoni would be inference from relatives such as Aequidens tetramerus rather than observation of this fish.
Breeding
Breeding has never been described in the wild, and the fish is obscure enough in the hobby that even captive reports are thin and second-hand. As a member of Aequidens proper, it is expected to be a biparental open-substrate spawner: the pair cleans a flat surface — a stone, a submerged log, or a broad leaf — and the female lays a clutch of adhesive eggs that both parents guard and fan, before moving the brood to pits and shepherding the free-swimming fry under continued joint care. Its type-relative Aequidens tetramerus spawns this way, laying on the order of a thousand eggs on stone or wood with both parents tending the young. Hobby sources give a smaller clutch of several hundred eggs for Aequidens rondoni, hatching in about two to three days and free-swimming roughly a week later — but those numbers trace to no documented spawning of the species and should be read as genus-typical rather than verified for this fish.
In the aquarium
Aequidens rondoni surfaces only rarely in the hobby, through specialist importers and German cichlid clubs, and dedicated keeper accounts are scarce. What follows leans on the confirmed habitat data — the soft, warm, acidic clearwater streams of the upper Tapajós-Juruena — the sections above, and the well-documented husbandry of its closest available relative, Aequidens tetramerus, as set out by Seriously Fish; where nothing specific to this fish exists, that is said plainly.
Tank: a pair does well in base dimensions around 100 × 15.5 in (roughly 40 US gal), generous for a fish topping out near 3.5–4 in. A breeding pair claims a sizeable territory, and cramped quarters bring chronic stress and torn fins. Keep the pair alongside dither fish, and 120 × 17.5 in is the more comfortable minimum.
Water: the habitat sets the numbers — soft water (up to roughly 8 °dH), pH 5.0–6.5, temperature 75–82 °F — and these are not mere preference. The upper Tapajós is a crystalline shield drainage with conductivity in the tens of µS/cm. Keeping the fish in liquid rock (tap water above ~150 µS/cm, or pH above 7) is the commonest mistake with obscure Amazonian acaras, and it risks chronic health problems, lateral-line erosion, and failure to colour up. Reverse-osmosis water blended down to about 30–50 µS/cm and buffered to pH 6 with alder cones or Indian almond leaves hits the target. Weekly changes of 30–50 % are essential; like all acaras, it is sensitive to dissolved waste and should never go into an immature tank.
Substrate and décor: use fine sand, since the fish forages just above the bottom and works mouthfuls of sand and debris in typical acara fashion — coarse gravel abrades the mouth over time. Add flat stones as potential spawning sites, driftwood or bogwood, and scattered dried leaf litter to echo the woody, leafy zone it lives in. Dim lighting suits it and lifts the iridescence in the flank scales, and floating plants help. Dense planting is unnecessary, but hardy low-light species such as Microsorum or Anubias tied to wood work well.
Filtration: an external canister sized for gentle, low-flow output matches the un-turbulent headwaters — position it so it does not throw a strong current, and fit a pre-filter sponge, as stray sand wears pump impellers.
Diet: in the tank it takes standard cichlid fare without fuss — sinking pellets or granules as a staple, plus frozen or live bloodworm, Artemia, and mosquito larvae. Vary it; the same dried food day after day invites nutritional deficiency. Feed two or three small portions a day rather than one large one.
Tankmates: outside breeding the fish is only moderately territorial, in line with the genus. Good companions are similarly sized soft-water South Americans that use other water layers — small characins (hatchetfish, pencilfish, smaller tetras), dwarf cichlids that will not contest the same benthic ground, and Corydoras that share the substrate without drawing sustained aggression. Avoid anything big enough to bully it, anything that needs hard alkaline water, and very small fish that a breeding pair might harass. Beyond an established pair, conspecifics are not recommended unless the tank is large enough for several territories.
Breeding in captivity: the genus account above covers the essentials. To trigger a spawn, condition the pair on live and frozen food, give them warm soft acidic water, and provide a clean flat surface. Once a pair bonds and spawns, the shared guarding is pronounced and the two should be treated as a unit; removing tankmates reduces interference. Precise triggers for this species have not been published, so treat successful accounts for Aequidens tetramerus and related acaras as the best available guide.
Difficulty: intermediate. Behaviourally the fish is undemanding, but the need for genuinely soft, acidic water is non-negotiable for a clearwater Tapajós endemic. Keepers without a reverse-osmosis unit or suitably soft tap water should leave it alone, and its scarcity in trade means sourcing and acclimatisation deserve care.
Conservation
The IUCN Red List assesses Aequidens rondoni as Least Concern, an evaluation made on 7 November 2018 by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio), Brazil's assessing body. The fish is reported common and abundant where it occurs, which carries the listing despite its restricted, single-basin range. The assessment notes that the basin has been reshaped by human activity — agriculture in particular, together with at least five small hydroelectric plants inside the species' extent of occurrence and more dams forecast — but judges these pressures no direct threat to the population as things stand. There is no CITES or CMS listing. For any narrow-range Amazonian endemic, the direction of the upper Tapajós — damming, deforestation, and agricultural expansion across the Chapada dos Parecis — is the backdrop to watch, even if none of it is yet documented as an active danger to this cichlid.