Fish

Corydoras

Corydoras · also called corydoras, cory catfish, cory

Least Concern → omnivorecarnivore Rainforest Rivers & wetlands

A corydoras is a small, tank-shaped catfish in a suit of bony armour, trundling along the bottom of a stream on three pairs of whiskers, sorting sand for food. It is one of the most kept aquarium fish in the world, it breathes air through its gut when the water goes bad, and it will only be happy in a crowd.

Overview

Corydoras are the genus Corydoras — around 170 species of small armoured catfish from the streams and floodplains of tropical South America. Instead of scales they have two rows of overlapping bony plates, plus locking fin spines and three pairs of barbels for tasting and feeling out food in the substrate.

Gut-breathing and shoaling

A corydoras can absorb oxygen through part of its intestine: it dashes to the surface, gulps air, takes the oxygen as it passes through, and vents the rest — which lets it survive warm, stagnant pools. It is also strongly social, foraging in groups of hundreds in the wild, so a lone one in a tank sulks; keepers give them groups of six or more.

Breeding in a T

Corydoras spawn in the “T-position”: the female holds eggs between her pelvic fins and puts her mouth to the male’s vent, apparently passing sperm through her gut to fertilise them, then sticks the eggs to plants and glass.

Status

Least Concern for most species, now largely farm-bred for the trade. A few corydoras confined to a single stream or headwater are threatened by deforestation, mining, and dams. Their fin spines carry a mild venom — a jab stings like a wasp for a few hours.

Corydoras: common questions

How many corydoras should I keep?

At least six of the same species, and more is better. Corydoras are shoaling fish that in the wild forage in large groups, and their behaviour and wellbeing depend on company. Kept singly or in twos and threes they tend to sit still, hide, and stop foraging actively, whereas a group of six or more spreads out across the bottom, feeds confidently, rests in loose piles, and often performs synchronised dashes to the surface for air. Mixing several different corydoras species in a tank looks fine but each species does best with its own kind, so a group per species is the ideal.

Why does my corydoras keep swimming to the top?

Occasional quick trips to the surface are completely normal. Corydoras breathe partly through their intestine, and they routinely dart up, gulp a bubble of air, and return to the bottom, more often in warmer water. Frequent, frantic surface gasping by all the corydoras at once is different and is a warning that the water is low in oxygen or poor in quality, from overstocking, a stalled filter, high temperature, or a build-up of waste, and it calls for a water change and better aeration.

Do corydoras clean the tank?

Only in a limited way. Corydoras are useful because they forage across the bottom and will eat food that has sunk and been missed by other fish, which helps stop leftovers from rotting. But they do not eat fish waste or algae to any real extent, and they should not be relied on as a clean-up crew or expected to live on scraps. They need their own sinking food, ideally after lights-out, and they do best over smooth sand or fine rounded gravel that will not wear away or damage their delicate barbels.

Are corydoras venomous?

Mildly, and only defensively. The stiff, locking spines at the front of the dorsal and pectoral fins carry a weak venom, and a person who is jabbed, usually while netting a fish or reaching into a transport bag, feels a sharp sting like a bee or wasp, with some swelling and aching that fades within a few hours. It is not dangerous to a healthy person, though as with any sting a rare individual may react more strongly. The spines and venom exist to make the little armoured fish an unpleasant mouthful for a predator.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 11, 2026. Last updated September 11, 2026.

Quick facts

Scientific name: Corydoras
Diet: A bottom forager that sifts sand and fine gravel through its mouth and gills to sort out food, taking worms, insect larvae, small crustaceans, and other invertebrates, plus fallen plant matter, and in the aquarium readily eating sinking pellets, wafers, and frozen foods
Armour and barbels: A corydoras is protected by two overlapping rows of bony plates down each side instead of scales, has sharp locking spines in the dorsal and pectoral fins, and carries three pairs of sensitive barbels around the mouth that it uses to taste and feel for food in the substrate
Breathing through the gut: Corydoras can absorb oxygen through a modified, thin-walled section of the intestine, and in poor water they dash to the surface, gulp a mouthful of air, take up the oxygen as it passes through, and expel the spent air from the vent, which lets them survive in warm, stagnant pools
A shoaling fish: Corydoras are social bottom-dwellers that live and forage in groups, sometimes of hundreds in the wild, and a lone or paired corydoras in an aquarium becomes withdrawn and inactive, so keepers house them in groups of six or more, ideally of the same species, where they feed and rest together
The T-position spawn: When breeding, the female cups a few eggs between her pelvic fins and positions herself at right angles to the male, mouth to his vent, in a T-shape, apparently drinking sperm that then passes through her gut to reach and fertilise the eggs she is holding, which she then sticks to plants or glass
Around 170 species: The genus Corydoras and its close relatives contain around 170 described species from across tropical South America, with many more awaiting names, and hobbyists track undescribed forms using a C-number cataloguing system while science catches up
Mild venom in the spines: The locking pectoral and dorsal fin spines carry a mild venom, and a jab from a corydoras, most often when netting one or when a bag is handled carelessly, produces a sharp, wasp-like sting and local swelling that passes in a few hours, harmless but memorable
Range: Fresh water throughout tropical and subtropical South America east of the Andes, from Colombia and the Guianas south to northern Argentina, in the shallow, sandy or muddy margins of streams, small rivers, floodplain pools, and swamps, often in soft, warm, slow-moving or still water among leaf litter, roots, and submerged wood.
Conservation status: Least Concern → ≈ Most corydoras are widespread across the rivers and streams of tropical South America and are assessed as Least Concern, and the popular aquarium species are now largely produced on fish farms in Southeast Asia and eastern Europe, which reduces pressure on wild stocks, though a large wild-caught ornamental fishery continues in parts of the Amazon and provides income that supports keeping rivers and forest intact. Some corydoras have very small ranges limited to a single stream or headwater system and are vulnerable to deforestation, agriculture, mining, and dam building, and a few of these restricted species are considered threatened in the wild