Taxonomy & naming
Few cichlids carry as tangled a nomenclatural history as the green discus. The genus Symphysodon was erected by the Austrian naturalist Johann Jakob Heckel in 1840 for the Heckel discus, Symphysodon discus; a second long-recognized species, Symphysodon aequifasciatus, was described by Jacques Pellegrin in 1904 from a small type series — one fish from near Santarém and two from the Tefé region. The name tarzoo entered the literature in 1959, when the American aquarist Earl Lyons published a short note in the hobby periodical Tropicals Magazine (volume 4, 'New blue discus electrify aquarium world') announcing a western form distinguished by red spots on the body and anal fin. The animals had been imported by Mike Tsalickis of the Tarpon Zoo, a Florida export firm with a collecting base at Leticia, Colombia — and Lyons coined 'Tarzoo' from the company's name.
Whether Lyons' note constitutes a valid scientific description has been disputed ever since. Leonard Schultz, revising the genus in 1960, dismissed tarzoo as a nomen nudum and replaced it with his own Symphysodon aequifasciatus haraldi. For decades only two species — discus and aequifasciatus — were generally accepted. The name was resurrected by Sven Kullander (1996) and then formally applied by Ready, Ferreira & Kullander (2006), whose mitochondrial-DNA study found a phylogeographic break in the genus: a distinct, red-spotted western lineage that they assigned to Symphysodon tarzoo. Crucially, that paper also designated the Santarém specimen (lectotype MNHN 1902-0130) as the lectotype of Symphysodon aequifasciatus, fixing that name on the central-eastern fish and leaving tarzoo as the available name for the western green discus; the authors selected a neotype for tarzoo, INPA 25960, an adult male from the Rio Jutaí in Amazonas, Brazil.
That is not the end of the matter. A near-simultaneous monograph by Heiko Bleher and colleagues (2007) reached a different arrangement — discus, aequifasciatus (the green fish), and Symphysodon haraldi (the brown-and-blue fish) — under which tarzoo falls into the synonymy of aequifasciatus. A separate critique by Martin Geerts (Cichlid Room Companion, 2011) argues the name tarzoo is not available at all, because Lyons wrote 'Symphysodon Discus Tarzoo' with the species-group words capitalized, contrary to the rules of the Code. Microsatellite work by Amado, Farias & Hrbek (2011) added yet another layer, recovering five evolutionarily significant units within the genus and aligning Symphysodon tarzoo with the green phenotype. The upshot for a reader today: FishBase, Eschmeyer's Catalog of Fishes, and the IUCN currently treat Symphysodon tarzoo as a valid species, but the boundaries and even the correct names within the genus remain genuinely contested. Whichever label one uses, the western green/red-spotted discus is a real, diagnosable biological entity; the dispute is about what to call it, not whether it exists. Common trade names include green discus, Tefé discus, and red-spotted green discus.
Morphology
Discus are among the most distinctive of all cichlids: the body is strongly laterally compressed and nearly circular in profile — the 'disc' that gives the genus its common name — with a small mouth, high dorsal and anal fins that sweep back to meet a short caudal peduncle, and a slow, deliberate way of moving through the water. Reported maximum size is about 6.5 in standard length, so a large adult is a hand-sized, coin-flat fish. The base color of Symphysodon tarzoo is an iridescent green to blue-green; what sets it apart from its eastern relatives is the scatter of rust-red, orange, or reddish-brown spots and short streaks across the flanks and onto the unpaired fins. In the genus's shared body plan, roughly nine faint vertical bars cross the sides, one passing through the eye and a darker one through the mid-body; these can intensify or vanish almost instantly with mood.
Color within wild populations is highly variable — a polychromatism, not a set of fixed local races. In a single Tefé-region catch one finds heavily 'Royal' fish, marked with bold horizontal blue-green striping running the length of the body, alongside individuals with many red spots, few spots, or none. These forms interbreed and live together; the spotting and striping are points on a continuum rather than separate populations. The clearest geographic difference is in shape rather than color: fish from the still water of Lago Tefé tend to be taller-bodied than their slimmer river counterparts. Sexual dimorphism is weak — mature males average a little larger with slightly thicker lips and more pointed fins — and reliable sexing usually waits until a pair forms and the genital papillae are visible at spawning.
Habitat
Symphysodon tarzoo is the discus of the western Amazon Basin, occurring in Brazil (state of Amazonas), Colombia, and Peru. Its range lies west of the Purus Arch — a low geological divide running roughly between Manaus and the Rio Bauana that separates it from Symphysodon aequifasciatus to the east; only a handful of tarzoo specimens have turned up from the Madeira downstream of the arch, and essentially no aequifasciatus upstream of it, making the boundary a textbook example of an Amazonian phylogeographic barrier. The species follows the whitewater and clearwater systems of the Solimões–Amazonas upstream of the Negro confluence, the classic collecting grounds being the Rio Tefé and Lago Tefé. The IUCN estimates an extent of occurrence of roughly 436,0 mi², though much of the Colombian and Peruvian range is poorly documented and the true distribution is likely wider.
The habitat is the seasonally flooded lowland Amazon: floodplain lakes, channels, and inundated forest where the water is soft and acidic. Discus are creatures of structure and shade rather than open river — they shelter among submerged root tangles, leaf litter, and especially the drowned crowns of fallen trees. These habitats swing dramatically with the flood pulse: annual water-level amplitude in the Tefé region runs between about 8 and 16 meters, peaking around April–June and ebbing in October, so the fish's world expands into the forest at high water and contracts to lake margins at low water. Whitewater floodplains around Tefé carry conductivities on the order of 80–200 microsiemens per centimeter, while the blackwater systems draining the surrounding terra firme are far more dilute and more acidic — the soft, tannin-stained, low-mineral water that discus keepers spend so much effort imitating.
Feeding
Despite a reputation in the hobby as a delicate specialist, the wild green discus is a generalist forager near the base of the food web, with an estimated trophic level around 3.1. Field study of western discus (Crampton, 2008, working on what was then called Symphysodon aequifasciatus, i.e. the western green fish) found a diet dominated by algal periphyton scraped from submerged wood and vegetation, fine organic detritus, plant matter, and small aquatic invertebrates — the small terminal mouth is well suited to picking and grazing biofilm rather than seizing large prey. Stomach-content studies of discus more broadly report a similar mix weighted toward algae, detritus, and plant material, with invertebrates a smaller fraction, and the balance shifting with the season.
Feeding tracks the flood cycle closely. At high water the fish disperse to forage alone or in small groups through the flooded forest, where inundated leaf litter and periphyton-coated wood are abundant and predators and food are spread thin. As the waters fall, the fish concentrate into large aggregations in the fallen tree crowns along lake margins, where shelter and feeding opportunity are compressed into a much smaller volume. This seasonal expansion and contraction of habitat — and the accompanying swings in dissolved oxygen, predator density, and parasite load — shapes much of the species' biology, including the timing of its reproduction.
Mating
Outside the breeding season discus are gregarious, shoaling fish — a fact often missed by keepers who buy them in pairs and end up with stressed, bullied animals. In the wild they gather in loose schools among submerged structure, and the species' aggression is mild by cichlid standards, expressed mostly as transient sparring rather than the relentless hostility of many Rift Lake cichlids. Reproduction begins socially, with the formation of bonded, monogamous pairs out of the larger group; a pair forms a lasting bond and then withdraws to establish and defend a small territory around a chosen spawning surface.
Sexual dimorphism is weak, so pair recognition in the wild and in the aquarium is behavioral rather than visual — the keeper typically only knows a true pair has formed when two fish stop sparring, begin moving and cleaning a surface together, and turn their aggression outward against the rest of the group. In the wild, the onset of pairing and spawning is keyed to the annual flood: courtship and territory formation ramp up as the waters rise, so that the demanding parental phase coincides with the food-rich, predator-diluted high-water forest. Among Amazonian cichlids this tight reproductive synchrony with the flood pulse is one of the green discus's defining traits.
Breeding
Breeding is biparental substrate spawning. A bonded pair clears a vertical or steeply sloped surface — in the aquarium a piece of slate, a broad leaf, or the side of a cone — and the female lays a row of adhesive eggs that the male fertilizes, both parents fanning and guarding the clutch. The eggs hatch in a few days; the parents move the wrigglers and, once the fry are free-swimming, comes the behavior that makes discus famous. Both adults secrete a thick, nutritious mucus from skin that hypertrophies for the purpose — 'discus milk' — and the fry swarm onto their parents' flanks and graze on it directly, flicking from one parent to the other as the adults take turns.
This mucus is essential to early larval growth; the parent–offspring relationship has been compared to mammalian lactation, and the genus shares the trait only with its close relative Uaru among the cichlids studied (Buckley et al., 2010). Over the following weeks the fry are gradually weaned onto free food. In the wild, spawning is timed to the onset of the flood season, so that young are well grown before the next low-water bottleneck; the species is an iteroparous partial spawner that can reach reproductive maturity within a year and breed repeatedly across a season. Both parents share the mucus-feeding burden, which is one reason discus are kept and bred as established pairs rather than as a male rotated between females.
In the aquarium
Discus are not beginner fish, and honest advice starts there. They are large, tall-bodied, slow-growing cichlids that need volume and stability: a group belongs in a tall aquarium of at least 75 gallons (about 75 US gal), and more for a full shoal. Because they are shoaling animals, they should be kept in groups of six and ideally ten or more — a single pair or trio tends to produce a dominant bully and chronically stressed subordinates, which in turn invites the disease problems discus are infamous for. The water must be warm, soft, and clean: keepers run wild-type discus toward the upper end of tropical temperatures (roughly 82–86 °F / 82–86 °F), with soft, acidic water and very low dissolved organics. Their sensitivity is less about hitting an exact pH number than about pristine water quality — discus are intolerant of accumulated nitrogenous waste and reward heavy filtration and frequent, generous water changes. This is the real reason for their reputation: the husbandry bar is high and unforgiving, not the fish's temperament.
Wild-collected Symphysodon tarzoo asks more than tank-bred discus, which have been selected over many generations to tolerate harder, more neutral water; imported green discus do best with the soft, mildly acidic, tannin-stained conditions that mirror the western Amazon, and benefit from being quarantined and treated for internal parasites on arrival. Tankmates should be peaceful and warmth-tolerant — small tetras and other soft-water characins, corydoras and other gentle catfish, and similarly mannered dwarf cichlids are the usual companions; tellingly, aquarists routinely keep cardinal tetras permanently at 82–86 °F alongside discus, a 'sauna' that suits the cichlid better than the tetra (AMAZONAS, May/June 2024). Boisterous or fin-nipping fish, and anything that won't thrive at discus temperatures, are poor matches. The most common mistakes are predictable: too few fish, a tank too short, water changes too small or too infrequent, and tankmates that outcompete these deliberate, unhurried feeders. Given space, numbers, warmth, and clean soft water, they are hardy and long-lived; deprived of any one of those, they decline quickly.
Conservation
Symphysodon tarzoo is assessed by the IUCN as Least Concern (Frederico, R.G. 2023; assessment based on a 2020 review). The rationale is its wide — if imperfectly mapped — distribution across the western Amazon and the absence of any documented severe threat, with the explicit caveat that data on its range, population size, and trends are thin and that further research is warranted. There are no population estimates; the species is described as not especially common or abundant where it occurs.
The one pressure the assessment singles out is the aquarium trade. Discus, alongside the cardinal tetra, have long been the flagship ornamental exports of the Brazilian Amazon (AMAZONAS, May/June 2024), and while most fish sold worldwide are now captive-bred, wild green discus from the Tefé region and elsewhere remain actively collected and exported, raising the prospect that localized overharvesting could become a threat even though it has not been demonstrated. More broadly, the western Amazon floodplain that this fish depends on faces the basin-wide pressures of deforestation, the disruption of the natural flood pulse by dams and climate change, and the degradation of the inundated forests and floodplain lakes that provide discus with food, shelter, and spawning sites. Because the species' reproduction is tightly keyed to the flood cycle and its habitat is the flooded forest itself, changes to that hydrological rhythm — rather than direct exploitation — may prove the more consequential long-term concern for the green discus.