Start with at least six corydoras, and make all six the same species. Corydoras are genuine group fish whose confidence comes from numbers of their own kind. In a proper group they work the bottom together in loose formation, rest in contact with each other, and stay active in the open. In twos and threes the same fish often hide for much of the day, and beginners then conclude that a lively species is dull.
Why the Same Species Matters
A group means six of one species, not six corydoras of six different kinds. Different corydoras species may share a tank peacefully, but they do not treat each other as the core group in the same way, and a single fish of a species is effectively alone however many cousins surround it. If you want two species, plan two groups and a tank that honestly fits both, rather than splitting the number and leaving each species short. Patterns make this mistake easy, because several species look similar at juvenile size in a shop tank. Ask for the scientific name of the fish you are buying and write it down, so additions later match the group you already own.
Fit the Group to the Tank by Choosing the Species
Do not solve a small tank by buying fewer fish. Solve it by choosing a smaller species. Corydoras species differ considerably in adult length, and the smaller kinds let a modest tank hold a full group with room to forage. Judge the fit by floor space, because these fish live on the bottom and a tall tank gives them little extra territory. A group also produces waste as a group, so the tank and filter need to be mature and cycled before the fish arrive, with capacity for six appetites from the first day rather than a load you grow into later.
Set the Bottom Up for a Group
Cover the bottom with smooth sand or fine, rounded substrate. Corydoras sift the substrate through their gills and push their barbels into it while feeding, and sharp or coarse gravel can wear those barbels down. Add cover at ground level, such as low plants, leaf litter, or a cave that several fish can share, arranged so the group can retreat without scattering into separate corners. Keep the flow gentle at the bottom and leave an open patch of sand where the group can gather and feed together, because that gathering is the behaviour the right setup rewards you with.
Add Them Together and Feed Them Directly
Add the group together rather than one or two at a time, so no fish spends its first weeks waiting for company and no newcomer joins an established pattern alone. Feed the corydoras directly with sinking food that reaches the bottom, in an amount the group clears steadily. They are sometimes sold as a cleanup crew, but leftover flakes rarely reach them in a tank with faster fish above, and a group fed only on scraps slowly loses condition. Watch at feeding time during the first weeks. The whole group emerging to forage together is the clearest sign that the number, the species, and the setup are right.
Grow the Group, Not the Guesswork
If the tank has room and the group thrives, you can add more of the same species later, quarantining new fish first and matching the scientific name exactly. Note losses and additions with dates, because group size drifts over the years and a group that has quietly fallen to three needs topping up for the remaining fish's sake. KinsLe's Pet Passport gives that record a permanent home, since the species name, the starting number, and each dated change can sit alongside the tank's water test notes and feeding routine.