The minimum is a group minimum
An adult corydoras catfish should have at least a 20-gallon tank, and that number only makes sense read as group housing. Corydoras live in shoals, and an adult kept alone hides, feeds tentatively, and behaves like a fraction of itself. Six adults of one species is the practical floor for normal behavior, and the 20-gallon minimum is sized for that group, not for a solitary fish in a big empty box.
Within that volume, choose footprint over height every time. An adult corydoras earns its living on the bottom, nosing through substrate, resting in loose lines with its shoalmates, and making short darts to the surface. A long 20-gallon tank gives the group a genuine foraging field; a tall tank of the same gallons gives them a chimney they will mostly ignore.
The floor itself is part of the size question
For an adult corydoras, substrate quality counts as living space. Sharp or coarse gravel wears down the sensitive barbels around the mouth, and a damaged barbel makes feeding harder and infection more likely. Smooth sand is the ideal floor: the group sifts it constantly, blowing small clouds as they hunt, and a sand bed effectively enlarges the useful territory because every inch is workable.
If sand is not possible, choose the finest, roundest gravel available and keep it scrupulously clean. Detritus trapped in coarse substrate rots exactly where an adult corydoras presses its face all day, and bottom water quality in a neglected gravel bed is far worse than a test of the open water suggests.
Leave the floor open. Low caves and plant cover belong at the back and edges, but the central area of a corydoras tank should stay clear so the shoal can gather, feed, and rest together in the open the way they naturally do after lights dim.
Height, lids and the surface dash
An adult corydoras regularly darts to the surface for a gulp of air, so the tank needs a little headroom discipline rather than height for its own sake. Fill to a level that lets that dash end in water, keep a fitted lid on the tank, and avoid startling shadows over the surface where possible. A 20-gallon setup with calm surface access suits the group better than a brim-full taller tank with a gap at the rim.
Water movement should reach the bottom gently. A filter that only stirs the top leaves the corydoras layer warm and stale, so aim some flow low or choose filtration that circulates the full depth without turning the floor into a wind tunnel the fish must brace against.
Feeding a group spread across 20 gallons
Six adult corydoras need food that actually reaches them. Flake eaten at the surface by faster tankmates starves the bottom politely, so feed sinking wafers or pellets after the main feeding, ideally near lights-out when the shoal activates. Scatter across two spots on the floor so bolder fish cannot guard the whole ration from one cave mouth.
Watch barbels and bellies at feeding time. In a well-sized adult corydoras tank, every fish shows intact whiskers, a gently rounded belly, and enough confidence to feed in the open rather than snatching and retreating.
Recording the setup before it changes
Tank size, group count, and substrate are exactly the details that get lost when a corydoras group is rehomed, moved house, or handed to a sitter who assumes the setup is standard. Writing them down once, accurately, prevents a well-meant downgrade later, and KinsLe Pet Passport holds this corydoras group's records in one portable packet that travels with the fish.