Discus

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: plant safe.

Feeding

Carnivorous-leaning omnivore that needs frequent small meals of high-quality food. Standard diet is discus-formulated pellets, frozen bloodworm, frozen brine shrimp, frozen mysis, and the traditional beef-heart-based mix that many breeders prepare (a controversial choice, since beef heart is not part of any wild discus diet and contributes heavy bioload, but widely used by serious keepers for conditioning and growth). New fish often refuse food for days or weeks during acclimation; offer small amounts of frozen bloodworm in low-stress conditions and accept some initial fasting as normal. Multiple small feedings spread through the day (three to five for juveniles, two or three for adults) work better than one large meal and reduce uneaten-food waste in a tank that already demands aggressive water-quality management. Slow, deliberate feeders that lose food races to faster tankmates; in mixed-species setups, drop discus food in a quiet corner first or use a target feeder.

Compatibility

• Demanding species with strict requirements: 28-32 C, very clean and soft water, frequent water changes, and a peaceful tank environment. Most beginner setups cannot maintain the conditions
• Keep in groups of five or more. Discus form a hierarchy and a smaller group leaves the bottom-ranked fish chronically bullied. Solo discus shows persistent stress behaviours and rarely thrives
• Tankmate compatibility is limited by the temperature requirement. Workable companions: cardinal tetras (parameters match exactly), rummy-nose tetras, Sterba's corys (the warm-tolerant corydoras), bristlenose pleco, and a few other warm-acidic species
• Common pleco is the classic killer pairing: large plecos rasp the thick fry-feeding slime coat off discus flanks, leading to wounds, ulcers, and secondary infection. Always bristlenose, never common
• Avoid fast feeders, nippy barbs, boisterous swimmers, and cold-preferring species. Angelfish pairings are sometimes attempted but the risk of disease transfer and angelfish territoriality makes it inadvisable
• Captive-bred selective strains (pigeon blood, marlboro red, blue diamond, leopard, etc.) are noticeably more forgiving than wild-caught fish. Start with farm-bred stock if new to the species; wild discus is genuinely expert-only

Origin and habitat

Large, deep-bodied cichlids of the Amazon basin in Brazil, Peru, Colombia, and western Venezuela. Johann Jacob Heckel described the genus Symphysodon and the type species S. discus in 1840 from specimens collected in the Rio Negro. The number of valid species has been debated for decades. From Heckel's description through 2006, two species were generally recognised (S. discus Heckel 1840 and S. aequifasciatus Pellegrin 1904) with several named subspecies including S. discus willischwartzi (Burgess 1981, phenotype abacaxi), S. discus tarzoo (Lyons 1959, phenotype green), S. aequifasciatus haraldi (Schultz 1960, phenotype blue), and S. aequifasciatus axelrodi (Schultz 1960, phenotype brown). A 2006 DNA-based revision by Ready and co-workers recognised two species (S. aequifasciatus and S. tarzoo, with tarzoo elevated from subspecies). A 2007 revision by Bleher, Stölting, Salzburger, and Meyer (Aqua 12:133-174) instead recognised three species (S. discus, S. aequifasciatus, and S. haraldi). The two papers swapped the names: what the 2006 paper called S. tarzoo, the 2007 paper called S. aequifasciatus, and what 2006 called S. aequifasciatus, 2007 called S. haraldi. most current ichthyological references resolve this as three species (S. discus, S. aequifasciatus, S. tarzoo), though the underlying taxonomy is still contested. Amado, Farias, and Hrbek (2011 PMC3147135 International Journal of Evolutionary Biology) provided the molecular reference point for current usage. By geography, S. discus (the Heckel discus, with characteristic prominent first, fifth, and ninth vertical bars) is restricted to the Rio Negro and tributaries; S. tarzoo (green discus) occupies the western Amazon upstream of the Purus Arch; S. aequifasciatus (the blue and brown forms) is in the central Amazon east of the Purus. All three are primarily blackwater fish, with S. aequifasciatus also showing up in clearwater and whitewater. Some hybridisation occurs between species, particularly S. discus and S. aequifasciatus, but the species are evolutionarily distinct. Family Cichlidae, tribe Heroini. The most extraordinary feature of the genus is the parental feeding behaviour: free-swimming fry feed exclusively on a thick cutaneous mucus secretion produced from the parents' flanks for roughly the first three weeks after hatching, and the secretion has been documented in peer-reviewed work (Sylvain et al. PMC5507859) as containing the microbial pioneers that establish the fry's own gut microbiota - a form of vertical microbiome transmission similar in concept to mammalian breastfeeding. The behaviour is widely referred to as 'discus milk' and is rare among aquarium fish: the closest equivalent is found in the related cichlid genus Uaru. IUCN classifies S. discus and S. aequifasciatus as Least Concern (2022 ICMBio assessments). Adults reach about 15 to 20 cm SL depending on species and condition. Wild specimens are exported in significant numbers from the Brazilian Amazon, but the trade is now dominated by selectively bred captive strains: pigeon blood, marlboro red, blue diamond, cobalt, snakeskin, leopard, white butterfly, and dozens of other named lines, mostly produced in Southeast Asian (Singapore, Malaysia, Hong Kong) and German breeding operations. Captive-bred strains are noticeably hardier than wild-caught fish and have somewhat relaxed water requirements.

Breeding

Pair-forming substrate spawner. A bonded pair cleans a near-vertical surface (driftwood, a breeding cone, a broad plant leaf, or the tank glass itself) and the female lays 100 to 300 adhesive eggs in rows; the male fertilises them. Both parents guard and fan the clutch. The famous parental feeding behaviour begins once the fry become free-swimming, typically about a week after egg deposition: the fry swarm onto the parents' flanks and graze on a thick, nutritious mucus secretion produced by the skin, called 'discus milk'. Peer-reviewed work (Sylvain et al. 2017 PMC5507859) documents this as the exclusive food for free-swimming fry during roughly the first three weeks post-hatching, and the secretion also transfers the parents' microbiome to the fry, seeding their gut bacteria. Parents alternate as the primary mucus source. Fry raised away from parents have noticeably lower survival and require labour-intensive substitute feeding. Breeding requires very clean, soft, slightly acidic water (pH 5.5 to 6.5, GH below 3 dGH), temperatures around 28 to 30 C, and a stress-free environment with stable parameters. Pairs form naturally within a group of subadults; forced pairing of single fish usually fails. Commercial discus breeders use bare-bottom tanks with daily water changes and strict hygiene to control the pathogen load. After three to four weeks the fry begin taking baby brine shrimp and finely powdered food alongside the mucus feeding, and parents wean them off the mucus by gradually increasing their swimming distance.

Common problems

Discus are widely regarded as the most demanding commonly-kept freshwater fish. Sustained high temperature (28 to 32 C) accelerates metabolism, accelerates the multiplication of any pathogens present, and means water-quality problems develop faster than in cooler tanks. Most serious keepers perform large water changes (50 percent or more) every one to three days, often with bare-bottom tanks for easier cleaning. Internal parasites are nearly universal in imported and many farm-bred discus, particularly Hexamita and other intestinal flagellates; a prophylactic deworming course with praziquantel and a metronidazole treatment for protozoa is standard practice for every new arrival. Stress shows immediately as dramatic body-colour darkening, clamped fins, hiding, and loss of appetite, and chronic stress quickly progresses to disease. The high-temperature requirement eliminates most potential tankmates: cardinal tetras, rummy-nose tetras, Sterba's corys, bristlenose pleco, and a small number of other warm-tolerant species are the main options. Common plecos must not be housed with discus: the pleco's slime-coat-rasping behaviour is particularly dangerous against the thick fry-feeding slime coat of discus, which the pleco finds especially attractive. Slow, deliberate feeders that lose food races to anything more active. Many serious keepers run species-only tanks because the demands of maintaining proper discus conditions are hard enough without juggling other species' needs. The captive-bred strains widely available today are more tolerant than wild-caught fish, but the species remains genuinely advanced-level.

Bioload

Further reading

• The actual cost of keeping a fish tank for a year
• Blackwater tanks: replicating acidic jungle streams
• Setting up a breeding tank