Taxonomy & naming
Krobia xinguensis was described by the Swedish ichthyologist Sven O. Kullander in 2012, in the journal Zootaxa (3197: 43–54), on material that had sat in museum collections for decades. The holotype — an adult male of 3 in standard length (INPA 35653) — was taken at Ilha de Babaquara on the rio Xingu near Altamira, in the Brazilian state of Pará, by L. Rapp Py-Daniel and J. Zuanon in 1990; paratypes collected by R.H. Lowe-McConnell, J.-P. Gosse and others reach back to the early 1960s, drawn from headwater streams and lagoons in Mato Grosso. The genus Krobia was erected by Kullander and Nijssen in 1989 for a handful of Guianan acaras formerly lumped in the catch-all genus Aequidens, and before its 2012 description this fish circulated in the trade and the German aquarium literature simply as Krobia (or Aequidens) sp. "Red Cheek" / "Xingu." Kullander diagnosed it from its only two congeners, Krobia guianensis and Krobia itanyi, chiefly by a deeper caudal peduncle (17.6–19.6% of standard length, against 15.6–17.4% in Krobia itanyi), a dark lateral band that runs all the way to the end of the dorsal-fin base, two bold dark stripes between the eyes, a gently curved suborbital stripe, and a small dark brown blotch on each side of the lower jaw. Morphologically Krobia falls in a clade with Andinoacara, Bujurquina and Tahuantinsuyoa, all sharing a furrow on the ceratohyal for the hyoid artery and a forward-slanting lateral band; molecular work (Musilová et al. 2008, 2009; López-Fernández et al. 2010) repeatedly recovered this species — entered as "Krobia sp. Xingu" or "Xingu orange spot" — but unsettled its exact placement, in some analyses pulling it close to the Guianan Aequidens potaroensis. No junior synonyms exist; the species has been valid since erection.
Morphology
This is a small cichlid. The type series spans 1.5–3.5 in standard length, and FishBase records a maximum of 3.5 in SL — so a large wild adult is roughly the size of the largest paratype. Aquarium and hobby sources often quote around 4.5 in, a figure that almost certainly reflects total length (including the tail) on well-fed captive fish rather than the standard length used in the description; either way this is a palm-sized fish, not a large one. The body is deep and ovoid in profile (body depth averaging just under half the standard length) with a short head and snout, a rounded caudal fin, and pointed soft dorsal and anal fins. Fin counts are consistent: XIV dorsal spines with 8–11 soft rays, three anal spines with 7–8 soft rays. In preserved specimens the ground colour is a drab whitish to pale brown crossed by seven faint vertical bars, a dark lateral band from the eye to the end of the dorsal base — thickened into a round black blotch over the mid-flank — plus the diagnostic paired interorbital stripes, a curved suborbital stripe, and the twin dark spots under the lower lip. Life turns this drab template vivid: the upper lip, lachrymal, cheek, opercular margins and pectoral axil flush orange to red, and the base of nearly every flank scale carries a short red or orange vertical streak — the source of the "Red Cheek" trade name. Sexual dimorphism is modest. Males grow somewhat larger, carry slightly longer soft dorsal, anal and pelvic fins, and show stronger colour; females are plainer with shorter fins. Keepers consistently report that the sexes are hard to separate reliably until a pair actually spawns.
Habitat
Krobia xinguensis is known only from the Xingu River drainage, a large clear-water river that drains the Brazilian shield and joins the Amazon from the south. Records run the length of the basin, from headwater streams and lagoons in Mato Grosso (the Suiá-Missu, Batovi, Tuatuari and Suiá-Missu systems around the upper Xingu) down to the type locality at Ilha de Babaquara near Altamira on the middle-to-lower river. Detailed in-situ habitat notes for the type material are thin, but the localities that are described point to a fish of shallow, structured margins rather than open river. Lowe-McConnell (1991) reported it from the córrego do Gato, a clear-water habitat of stream, pool and small lake with abundant vegetation and aquatic invertebrates; Stawikowski (2007) figured a small rocky stream at São Félix on the middle Xingu as habitat. Hobby accounts consistently describe clear, warm, well-oxygenated shallows with moderate flow, scattered plants and submerged wood. The Xingu is a classic clear-water river — more transparent and less acidic than a true blackwater tributary — so wild fish experience soft, warm, mildly acidic to near-neutral water rather than the extreme low pH of the Rio Negro. Recorded as benthopelagic, the species occupies the lower and middle reaches of the water column over sand and leaf litter. Documented habitat depth is shallow, on the order of the first metre or two of these marginal streams, pools and lake edges.
Feeding
Krobia xinguensis is an opportunistic benthic omnivore. FishBase places it at a trophic level near 3.3, and the dentition Kullander described — an outer row of stout, blunt unicuspid teeth backed by several rows of smaller inner teeth, over a short, wide lower pharyngeal plate with coarse central teeth — is the generalist toolkit of a fish that takes small invertebrates and processes a range of bottom foods rather than specialising. In the wild it forages over sand and leaf litter for aquatic insect larvae, micro-crustaceans, other small invertebrates, plant matter and organic detritus, the mixed diet typical of acaras in vegetated clear-water margins. Despite its modest body size it is a notably greedy feeder: keepers describe it as "all mouth," with one reporting that a small group cleared four adult cardinal tetras within hours of introduction — a useful caution that the gape is large relative to the fish, and that slim, small companions are at risk even though the species is otherwise peaceable. In the community it functions as a mid-water and benthic forager, working the lower levels of the tank for anything it can swallow.
Mating
Outside of breeding Krobia xinguensis is a loosely social, generally peaceable fish that in the wild is found in loose aggregations, which is why keepers are advised to grow on a small group of six to eight and let pairs form naturally rather than forcing two unfamiliar adults together. Once a pair bonds, courtship follows the familiar substrate-brooding acara script: the male intensifies his red head and flank colour and flares his fins to solicit the female, the two trade vibrating displays that bring up the female's flank barring, and together they select and clean a spawning surface — typically a flat stone or a cleared patch of substrate — and establish a defended territory, driving other fish out of the immediate area. Pair bonds appear comparatively stable and the species is not notably violent toward its own kind by cichlid standards; multiple keepers describe it as about as easy-going as a keyhole cichlid and report pairs spawning peacefully alongside small companions, even dwarf Apistogramma. The main behavioural caveat is the appetite rather than the temperament: a spawning pair is territorial but rarely dangerous to tankmates too large to be eaten.
Breeding
Krobia xinguensis is a biparental, open/substrate-spawning cichlid, and it breeds readily — a welcome contrast to the difficult tunnel-nesting acaras. The wild reproductive biology has not been documented in detail, so the clutch figures come from captivity and from genus-typical behaviour: a female deposits roughly 200–500 adhesive eggs on a cleaned flat stone or hard surface, the male fertilises them, and both parents fan and guard the clutch and the defended territory around it. Eggs hatch in about 2.5–4 days depending on temperature, after which the parents move the wrigglers among shallow pits in the substrate, shifting them from site to site until the fry are free-swimming roughly five to seven days post-hatch. Both parents continue to escort and defend the fry afterwards. Captive breeders consistently describe the species as easy and even prolific — "they breed like crazy" and "readily breed without effort" are typical reports — and pairs have spawned across a wide chemistry range, from soft water around 140 µS/cm to distinctly hard water of 200+ ppm at temperatures near 81 °F. Fry are straightforward to raise on newly hatched brine shrimp (Artemia nauplii) and finely crushed flake. No elaborate trigger is needed; clean, warm, stable water and a conditioned pair are usually enough.
In the aquarium
Krobia xinguensis is an intermediate-level fish that is undemanding in most respects but only intermittently available — it entered the hobby as Krobia sp. "Red Cheek" around the mid-2000s, was formally named in 2012, and still moves mostly as wild imports and occasional hobbyist-bred batches rather than as a trade staple. A pair is comfortable in a moderately sized tank in the region of 30 US gal, but a group of six to eight — the best route to a natural pairing — wants a footprint of 55 US gal or more so that subordinate fish can keep their distance. The tank should be biologically mature before the fish go in: like many clear-water acaras, this species is sensitive to deteriorating water quality, and keepers stress generous filtration plus regular partial water changes (30–50% on a weekly-to-fortnightly schedule is the common regime). Match the biotope to a structured clear-water margin: a soft sand substrate it can forage over, plenty of cover from driftwood, roots, rockwork and sturdy or potted plants, areas of moderate flow broken by calm resting spots, and a flat stone or two as a potential spawning site.
Water chemistry is flexible. The natural Xingu environment is soft, warm and mildly acidic, and a maintenance range of about pH 5.5–7.5 (trending toward 6.0–7.0) at 75–84 °F suits it well — but this is not a soft-water specialist, and captive fish have been kept and bred successfully in distinctly hard, near-neutral tap water (one breeder reported success around 240 ppm), so clean, stable water matters more than hitting a precise pH. Feeding is easy: it takes quality sinking pellets, flake and granules readily, and a varied menu of live or frozen bloodworms, brine shrimp and the like, plus some vegetable matter, keeps colour and condition at their best. The one real pitfall is its appetite relative to its peaceful reputation — the gape is large, and slim small fish such as cardinal or neon tetras can and do get eaten, so dither and companion species should be deep-bodied or too big to swallow (larger tetras, hatchetfish, rainbowfish, peaceful barbs, headstanders, Corydoras, peaceful plecos and similarly mannered small-to-medium cichlids). Otherwise it is a calm, attractive community cichlid that rarely troubles tankmates too large to eat; it should not be mixed with Rift Lake cichlids, whose higher baseline aggression, different water needs and unreadable colour signals make for a poor and stressful match.
Conservation
The IUCN Red List assesses Krobia xinguensis as Least Concern, in a regional assessment of Brazilian freshwater fishes coordinated by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio) and a large panel of Brazilian ichthyologists — the assessment was carried out in 2018 and published in 2022. It carries no CITES or CMS listing. The species is distributed the length of a large drainage and is not flagged as facing a population-level threat. That said, its entire range lies within a single basin under acute and well-publicised pressure: the Belo Monte hydroelectric complex on the lower Xingu has dewatered and re-engineered a long reach of the river (the Volta Grande), and dam construction, deforestation and other land-use change continue to alter flow, connectivity and habitat across the basin. Hobbyists have noted the impact anecdotally, debating whether wild Red Cheek Krobia remain readily collectable below the dams. None of this is documented as an immediate danger to this particular cichlid — as a fairly adaptable, widely distributed clear-water fish it is far less exposed than the Xingu's many narrow-range rheophilic endemics — but the basin-wide pressures are the backdrop against which its long-term status should be read.