Handling Corydoras: The Net, the Bag, and Knowing Which Cory You Bought

A corydoras is never picked up, but it is caught, bagged, carried home and worked around every time the water is changed. Each of those moments is shaped by what the fish is: an armored catfish with locking, venom-bearing spines and a defense that can poison its own water.

The Spines That Lock

Corydoras carry a stiff, sharp spine at the leading edge of each pectoral fin and of the dorsal fin. Catfish spines like these can be locked upright when the fish is threatened, which makes the fish harder for a predator to swallow and harder for a keeper to get out of anything it snags on. Corydoras belong to the armored catfish family Callichthyidae, which evolved venom glands of its own: a 2009 survey of venomous catfishes counted 182 to 194 callichthyid species as presumed venomous and found a venom peptide in extracts from several Corydoras species. A catfish spine in a human hand mainly causes pain and swelling where it went in. The spines are the reason a cory is never caught bare-handed.

Catching One Without a Net

Net mesh is where locked spines snag. A cory caught in a net erects its spines, the spines catch in the mesh, and the fish struggles against the mesh, which can damage its fins. A rigid plastic container, a food tub or a deli cup, does the job without mesh: lower it into the tank, herd the cory into it against the glass, and lift it out still in water. For carrying fish any distance, a small plastic bucket or food container with a well-fitting lid is safer than a bag, because the spines cannot puncture it.

When a bag is unavoidable, use strong bags, doubled, and tape the corners back so the base is rounded. The pointed corners are where spines puncture a bag.

The Bag, the Foam, and Arrival

A stressed cory releases a fluid from the base of its pectoral spine or from its gills. In the small, closed volume of a transport bag, that fluid can kill the fish that released it and every fish sharing the bag. The warning sign is a little foam on the surface of the bag water. If a bag foams at the shop, ask for the fish to be rebagged in fresh water. A careful seller catches cories into a temporary container first, lets them release whatever they are going to, then catches them again into clean water for the trip.

The toxin is one more reason cory bag water never reaches the tank. Float the bag for temperature, then lift the fish out in a container, following the float-and-transfer routine for any new fish, and pour the bag water away.

Water Changes Around a Bottom Feeder

Cories spend the day on the sand, which puts them exactly where the siphon works and where new water lands. Poor water quality is a suspected cause of barbel erosion, so the weekly change matters more for this fish than its size suggests. Move the siphon slowly so the group can shift out of its way, and run new water in over a rock or a plate so it does not blast a crater in the sand where the group rests. The replacement water goes in conditioned and at tank temperature, on the schedule the water changes guide sets by nitrate reading.

Dwarf or Standard: Knowing Which You Bought

Juvenile corydoras of very different adult sizes sit in the same shop tanks at the same small size. The pygmy cory, Corydoras pygmaeus, reaches about 30 mm (1.2 inches) standard length and, unlike most of its relatives, spends much of its time swimming in midwater. It is very peaceful and easily intimidated by larger tankmates, and it looks and acts its best in a group of at least 6, preferably 10 or more. A standard species such as Corydoras sterbai reaches about 65 mm (2.6 inches) and lives on the bottom.

The difference decides the tank. A dwarf species can live in something as small as 10 gallons, while a school of six standard cories needs the 20-gallon long. Check the scientific name on the shop label before buying the tank, not after. Whichever it turns out to be, the group is six of one species at minimum: a lone or paired cory hides, darts and goes off its food.

The Corydoras Catfish encyclopedia profile covers the species itself, and a four-way roundup sets it beside three very different pets in Jackson's Chameleon, Canary, Giant Millipede & Corydoras Catfish compared.

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Sources & Further Reading