Taxonomy & naming
Corydoras parallelus was described by the American ichthyologist Warren E. Burgess in 1993, in the magazine Tropical Fish Hobbyist, from the Rio Içana in the Rio Negro drainage of Amazonas, Brazil. The epithet parallelus refers to the two long, nearly parallel dark stripes along the body that give the fish both its scientific name and its common one, the 'two-line' or 'duplicate' cory; it also carries the hobby code C002.
It was described in, and long held within, the genus Corydoras (family Callichthyidae, subfamily Corydoradinae). The 2024 phylogenetic revision of the subfamily by Dias, Tencatt and colleagues split that oversized genus into several genera, and the two-line cory falls within the resurrected genus Hoplisoma, which holds the bulk of the short-snouted Rio Negro corys. Its current valid combination is therefore Hoplisoma parallelum, the name now used by FishBase and Planet Catfish; the aquarium trade still says 'Corydoras parallelus,' so we keep that as the heading while recording the new placement. Catalog of Fishes treats parallelus Burgess, 1993 as a valid species.
Morphology
The two-line cory is a small-to-medium short-snouted Hoplisoma, reaching about 2 in in standard length, with the deep, compact body and twin rows of bony scutes characteristic of the subfamily. Its defining feature is its pattern: a clean, pale-silvery to greyish ground crossed by two long, dark, roughly parallel horizontal stripes — one running along the upper flank and a second lower down — with a dark mask through the eye. This bold, simple striping is what separates it at a glance from the many spotted and reticulated corys of the same waters and from superficially similar gold-shouldered Rio Negro species.
The head is rounded with three pairs of barbels, and the dorsal and pectoral fins bear the usual stiff, locking spines. As in essentially all corys, females grow larger and notably rounder than males — best judged from above — and mature males tend to have more pointed fins.
Habitat
Corydoras parallelus is endemic to the upper Rio Negro basin of Brazil, with the type locality on the Rio Içana, a left-bank tributary, and records through the upper Negro drainage. This is blackwater country: tea-coloured, tannin-stained water draining ancient, nutrient-poor sands, characteristically very soft and distinctly acidic, often with pH well below neutral, and warm year-round.
The fish lives over sand and submerged leaf litter along the margins and tributaries of these blackwater rivers and streams. FishBase and hobby sources give an aquarium temperature range of roughly 75–81 °F, consistent with the warm equatorial lowlands it comes from. The soft, acid chemistry of its home waters is the key point for anyone keeping it: this is a genuine softwater fish rather than one of the broadly tolerant trade corys.
Feeding
No formal wild-diet study has been published for the two-line cory, but it is a typical benthic corydoradine and forages as its relatives do, sifting soft substrate and leaf litter with its barbels to take small invertebrates, insect larvae, microcrustacea, worms and organic detritus and expelling the cleaned sediment through the gills. In the aquarium it readily accepts sinking tablets, granules and crushed flake, together with live and frozen foods such as bloodworm, Daphnia, Tubifex and brine shrimp, which are also used to bring a group into condition for spawning.
Mating
Spawning of this particular species is not well documented, but corydoradine courtship follows a single well-studied template. They are egg-laying shoalers with no pair bond: a ripe female cleans a spawning site, the group becomes active, and a male takes up the 'T-position' at right angles to her head. Sperm is thought to pass rapidly through the female's gut to fertilise the few eggs she holds in a pouch formed by her clasped pelvic fins, which she carries to the cleaned surface and presses into place — the cycle repeating a few eggs at a time until the clutch is complete.
Breeding
The two-line cory is regarded as a softwater aquarium fish and is bred only occasionally; there is no widely published, detailed spawning account specific to it, so the following is by analogy with its blackwater relatives. Conditioning a group well and then performing a cool, large water change with soft, acidic water to mimic the rainy season is the standard trigger for corys of this type. They scatter adhesive eggs onto plants, wood and glass and give no parental care, so the eggs or the adults are separated to prevent predation. Soft, acid, scrupulously clean water for the eggs and very fine first foods for the fry are the obvious requirements for a Rio Negro blackwater species.
In the aquarium
C002 is a desirable but not everyday cory, prized for its clean two-striped pattern, and it is best suited to a mature softwater aquarium. It is a shoaling species and should be kept in a group of six or more, where it is far more confident and active than in ones or twos. A soft sand substrate is effectively mandatory — corys feed by sifting, and sharp or chipped gravel abrades and infects their barbels — and the tank is best furnished with wood, leaf litter and shade to echo its blackwater home.
The single most important husbandry point is water chemistry: this is a soft, acidic-water fish and does best in well-filtered, well-oxygenated water on the soft, low-pH side, kept warm (about 75–81 °F) with gentle flow and regular changes. It is entirely peaceful and an excellent companion for the small tetras, pencilfish and dwarf cichlids of the same blackwater communities, and should never be kept with anything large enough to bully it. As always with corys, lift them in a cup rather than a mesh net to avoid snagging the fin spines, and transport them carefully, since a stressed cory can release skin toxins that are dangerous in a small volume of water.
Conservation
Corydoras parallelus has not been assessed by the IUCN and carries no Red List category, so its conservation status is formally unknown. It is a range-restricted endemic of the upper Rio Negro. The Rio Negro remains one of the better-preserved large river systems of Amazonia, and the ornamental fishery there is, in places, a model of low-impact, livelihood-supporting collection rather than a threat. Even so, a narrowly distributed blackwater endemic is inherently sensitive to habitat change — deforestation, water-quality shifts and over-collection of a localised population — and the species would benefit from a formal assessment to establish its status.