Taxonomy & naming
Charles Tate Regan named this fish Heterogramma borellii in 1906, working from specimens the Italian zoologist Alfredo Borelli had collected in Brazil; the surviving syntypes are held at the Natural History Museum, London (NHMUK 1900.4.14.12-14). The type locality is 'Carandasiñho' — today written Carandàzinho — in the rio Paraguay basin of Mato Grosso, Brazil. When the genus Apistogramma was later adopted for these dwarf cichlids, the species became Apistogramma borellii and the original name Heterogramma fell away.
Few dwarf cichlids carry a longer synonymy, a by-product of how variable and how heavily traded this one is. Heterogramma ritense Haseman, 1911, described from Bolivia, and Heterogramma rondoni Miranda Ribeiro, 1918, from Brazil, are both regarded as the same fish. So is the trade's long-time favourite, Apistogramma reitzigi — a name with a tangled past, published more than once in 1938 (by Mitsch and, separately, credited to Ahl) alongside Apistogramma aequipinnis Ahl, 1938. Hobbyists once split the two by colour, treating 'reitzigi' as the yellow aquarium strain and borellii as the blue wild fish, but the two are now understood to be one polychromatic species, with reitzigi a junior synonym.
In the working framework Mike Wise maintains for the genus, Apistogramma borellii anchors its own small unit, the Apistogramma borellii complex, placed within the large Apistogramma regani lineage but standing apart from the rest — a 'non-allied' species, in many respects a deeper-bodied, higher-finned offshoot near the Apistogramma macmasteri group. Two features of its sensory anatomy are genuinely diagnostic: the infraorbital pores are reduced to three (an 'advanced' character), and the anguloarticular sensory canals are usually missing altogether — a trait it shares with just one other Apistogramma, Apistogramma trifasciata, another rio Paraguay fish. It carries the DATZ aquarium-trade code A234, with related or possibly conspecific local forms numbered A103, A104 and A105.
Morphology
This is a small, deep-bodied, strongly compressed dwarf cichlid, and its reported maximum size depends on which source you read. Seriously Fish gives about 2.5 in standard length for males and 2 in for females, while FishBase — following Kullander's 2003 checklist — carries a much smaller 1.5 in SL. The gap reflects different reference specimens rather than a real disagreement; most aquarium males mature in the 2–2.5 in range, with females noticeably smaller. The body is tall and slab-sided, topped by a high, sail-like dorsal fin and finished with a rounded to faintly spade-shaped tail. The leading dorsal membranes are not drawn into the long banners some relatives show, which gives the fish its smoothly arched, 'umbrella' outline.
Colour is the species' calling card, and it is famously plastic. Wild and well-kept males glow pale to sky-blue, often washed yellow on the cheeks, throat and fins and flecked red or orange on the gill covers; a dark zigzag lateral band usually shows only on the rear half of the flank, and the full set of vertical bars appears mainly under stress or in display. Females run smaller and plainer at rest but turn an intense lemon-yellow when guarding eggs or fry, marked with a dark lateral spot and black pelvic fins. The dimorphism is clear — males larger, brighter and longer-finned than females. On top of this natural range the trade has fixed several bred lines, the high-blue 'opal' form and various 'yellow' strains chief among them.
Habitat
Apistogramma borellii belongs to the Paraguay–Paraná system of south-central South America. It ranges through much of the rio Paraguay and the lower rio Paraná, from the southern Mato Grosso of Brazil down through Paraguay to about the city of Corrientes in northern Argentina, with further populations in tributaries of the rio Uruguay; its native-range countries are Brazil, Paraguay, Argentina and Bolivia. No other Apistogramma reaches so far south, into genuinely subtropical latitudes — the geographic fact behind the species' unusual cold tolerance.
Within that range it keeps to slow, shallow, structured water: creeks, weedy margins, backwaters and small tributaries rather than open river. The biotope is the familiar dwarf-cichlid mix — soft substrate, leaf litter, submerged wood and roots, plenty of low cover, and the dark crevices the fish adopts as spawning caves. Recorded chemistry is broad and locality-dependent, spanning roughly pH 5.0–8.0 and a wide hardness range on the order of 18–268 ppm (about 5–19 °dH), part of why the species settles in so readily. Temperature is what sets it apart. Most sources give a working band of about 68–79 °F, and FishBase classes the fish as subtropical; field accounts describe borellii — in full yellow breeding dress, no less — being netted from water far colder than any Amazonian apisto would abide, and keepers routinely run it heaterless at room temperature. It is, in effect, the cool-water specialist of a warm-water genus. Its reach extends into the middle and lower Río Uruguay too, where it turns up on both banks — in Rio Grande do Sul (Brazil), Argentina and Uruguay (Krause Lanés et al. 2010; Loureiro et al., New records of freshwater fish for Uruguay) — giving the species a genuinely four-country range across the La Plata system.
Feeding
In the wild Apistogramma borellii is a small, bottom-oriented micropredator. FishBase records its diet as worms, crustaceans and insects, and Seriously Fish calls it primarily carnivorous, feeding chiefly on benthic invertebrates — the tiny crustaceans, insect larvae and interstitial fauna it picks from sand, leaf litter and biofilm. Its estimated trophic level is around 3.3, placing it among the small invertivores of its community. Like most apistos it forages by working the bottom, mouthing and sifting fine substrate and detritus and spitting out what it cannot use rather than digging large pits.
In the aquarium that invertivore bias carries over, but the fish is undemanding. Live and frozen foods — Artemia, Daphnia, chironomid larvae (bloodworm) — are taken eagerly, and most specimens learn to accept good dry foods, generally preferring sinking pellets to flake. The decomposing leaf litter and microbe films of a naturalistic tank also seed a useful secondary food supply for foraging fry.
Mating
Apistogramma borellii is a cave-spawning dwarf cichlid built around a female-defended nest, like its congeners, but socially it sits at the gentler end of the genus — one of the least aggressive apistos and one of the easiest to pair. In the wild and in roomy aquaria it leans toward a loose polygynous, or harem, arrangement: a single dominant male holds a territory that overlaps the smaller territories of several females, each guarding her own cave. In an ordinary tank it is usually kept and bred as a single pair. Males set up over suitable spawning cavities and advertise with spread fins and heightened blue-and-yellow colour, while a ripe female signals readiness by flushing the vivid lemon-yellow that deepens once she spawns.
From there the sexes divide labour sharply. Once a female accepts a cave and spawns, she takes over the brood almost entirely and her tolerance for the male collapses; in small aquaria she can turn intensely aggressive toward him, and keepers often pull the male out to spare him a battering. This female-centred, cave-based system — not any lasting biparental bond — is the heart of the species' breeding behaviour.
Breeding
Breeding follows the classic Apistogramma pattern of a maternal cave spawner. The female cleans the ceiling of a dark cavity — a rock crevice, a hollow under wood, or in the aquarium an inverted flowerpot or coconut shell — and fastens her eggs there for the male to fertilize; FishBase puts a typical clutch at about 50–70 eggs. After that the work is essentially all hers: she tends and fans the eggs, defends the cave, and once they hatch shepherds the larvae and then the free-swimming fry, leading the tight school out to forage and herding it back into cover at any hint of danger. The male's part shrinks to guarding the wider territory, when he is tolerated at all.
Among aquarists the species is famous as an easy first apisto to breed, largely because it forgives so much in water chemistry and temperature. Soft, slightly acidic, warmish water and plenty of cave sites improve the odds, but borellii will often spawn in harder or cooler conditions that defeat fussier relatives. As in most apistos the brood sex ratio is temperature-sensitive, so the rearing temperature shapes how many males and females result. The fry are small and start best on infusoria and newly hatched brine shrimp, with the leaf-litter and biofilm of a planted, blackwater-style tank supplying extra grazing as they grow under the female's guard.
In the aquarium
Apistogramma borellii is probably the most forgiving member of its genus for a beginning dwarf-cichlid keeper, and the reputation is earned. The defining advantage is water chemistry: where most Apistogramma truly demand soft, acidic, warm water, this species will spawn and raise fry in tap water that would frustrate its Amazonian cousins.
Water: published figures bracket pH 5.0–8.0 and hardness to around 19 dH, and the fish has bred at both extremes; in practice moderately soft to medium-hard water around pH 6.5–7.5 suits it, and keepers in hard-water areas can use unconditioned tap water without the peat filtration or RO top-up other apistos need. Temperature is the other latitude — a subtropical fish, borellii is content in the low-to-mid twenties (°C) where most congeners want 79–82 °F, so temperate-climate keepers often run it heaterless at room temperature. Treat 68–79 °F as the working range and avoid sustained spells above 82 °F.
Tank: a single pair does well in about 23.5–27.5 in / 24 inches, while a harem of one male and two or three females becomes realistic at 35.5 in or more. Aquascape for the natural habitat — fine sand, a scatter of dried leaves (Indian almond, oak) for a biofilm-rich litter layer, and several caves such as half coconut shells, inverted terracotta flowerpots or hollow bogwood. Where more than one female is present, line-of-sight breaks between their territories matter; without them the male's attention falls too hard on one fish. Dense planting is optional but helps, and plants grow easily in the cooler water this species prefers.
Tankmates: a few dither fish — small, active, upper-water species such as pencilfish, tetras or Corydoras — genuinely earn their place, signalling that the tank is safe and drawing the male off when a guarding female wants him gone. Apistogramma borellii is gentle by apisto standards but is still a small cichlid that holds territory and punishes persistent intruders, so avoid other cave-holding cichlids of the same body scale, and keep it clear of fin-nippers and large, boisterous species.
Common mistakes: most trouble comes, paradoxically, from over-caution. Keepers used to delicate Amazonian apistos over-soften and over-acidify the water, or crank the tank to 82 °F on generic apisto advice, and the fish sulks or shows suppressed immune function in response. Given the modest essentials — some cover, a cave or two, cool-to-warm water, a few dither fish and no serious competition for space — this is among the easiest of all dwarf cichlids to keep well, and a lemon-yellow female leading a cloud of fry around a flowerpot cave is one of the most accessible pleasures in small-tank cichlid keeping.
Conservation
The IUCN Red List assesses Apistogramma borellii as Least Concern, in an assessment dated 28 October 2020 and published in the 2021 update (by Frederico). The species is widespread across the Paraguay–Paraná basin, lives in a range of habitat types, and faces no documented range-wide threat; it holds no CITES or CMS listing and rates as low in fishing vulnerability. Its sheer adaptability — the broad tolerance of pH, hardness and, above all, temperature that makes it such a good aquarium fish — also cushions it against the kinds of disturbance that press harder on narrowly specialised dwarf cichlids.
Even so, that is a basin-scale judgement, not a promise for every local population. The lowland waters of the upper Paraguay and Paraná carry the region's usual pressures — agricultural expansion and runoff, wetland drainage, and habitat alteration in the Pantanal and its surrounds — and the ornamental trade still draws on wild stock in parts of the range, though the fish is also bred in very large numbers in captivity, which eases collecting pressure. For now the picture is reassuring: a common, hardy, widely distributed cichlid in no apparent danger, valued enough by hobbyists to be propagated worldwide.