A Second Corydoras Shares Floor, Not Territory
A second corydoras catfish does not need a separate half of the tank, because corydoras are shoaling bottom dwellers that rest, forage and travel together rather than defending personal ground. What the pair consumes is floor area: enough open sand for both fish to sift side by side without constantly bumping shelters or each other. In a tank that was already only just big enough for one, the honest answer is that a second corydoras needs a larger tank, with a longer, wider footprint mattering far more than extra height the fish will rarely use except for quick trips to the surface.
Shelters and Resting Spots: Add at Least One
Two corydoras sharing one cave works on the first night and frays quickly after, so add at least one extra resting place when the second fish arrives. The pair will often choose to rest together anyway, but the choice is what keeps it peaceful; a corydoras that is nudged out needs somewhere shaded and low to go. Give the pair two or three low shelters of similar quality, placed close together on the sand, rather than one roomy cave and one exposed gap under a plant. Equal options stop the subtle competition that shows up as one fish always resting in the open.
Feeding Provision for Two Corydoras
A second corydoras needs its own reliable share at the bottom, so widen the feeding plan rather than simply dropping more food in the same corner. Use two feeding patches on open sand, far enough apart that each fish can settle at one, and offer enough sinking food for both to eat without hurrying. Watch the first several feedings fully: a confident corydoras will park itself over a wafer while a shyer one circles, and that circling is your signal to spread food over a wider area. Remove uneaten food once feeding winds down so the extra rations do not foul the sand the pair sleeps on.
Filtration, Water Changes and the Real Extra Cost
The invisible resource a second corydoras consumes is waste capacity. Two fish produce more waste on the floor where they live, so filtration should turn the water over comfortably and the water-change routine should be sized for the pair from the start, including gentle siphoning of the sand around feeding patches. Test the water more often during the first weeks together, since a slow drift in quality shows up in corydoras as faded colour, lethargy or worn barbels before anything dramatic happens. If problems persist despite clean water and steady feeding, an aquatic veterinarian can advise on what else may be affecting the pair.
Plan for a Group, Not Just a Pair
Two corydoras are company, but this is a fish that relaxes in a shoal, so the wisest extra-space calculation plans beyond the pair. Floor space, shelter count and filtration chosen for a small group from the outset cost less reshuffling than three separate upgrades as each fish arrives. Sketch the final group size the tank can genuinely hold, check the footprint against it honestly, and only then add fish gradually so the filter matures with each arrival. Writing that stocking sketch into KinsLe with Family Sharing stops a well-meant surprise purchase at home from overloading a tank the whole household thought was settled.