Corydoras are sold as the cheerful vacuum cleaners of the aquarium floor, and that billing sets new owners up for the mistakes that follow. A cory is not a janitor and not a solitary fish: it is a social, bottom-feeding catfish with specific needs for company, soft ground and food of its own. Get those three things right and a cory group is hardy and entertaining for years; get them wrong and the fish fade quietly at the bottom of the tank.
Mistake one: one cory is a lonely cory
In the wild, corydoras live in shoals that can number in the hundreds, resting flank to flank and foraging as a loose, bustling group. A single cory in a home tank behaves like a fish expecting to be eaten: it freezes in a corner, hides behind the filter and barely feeds. The fix is simple and non-negotiable — keep at least six of the same species together. Six bronze corys will school, rest and sift as a group; six fish of six different species mostly will not, because each kind looks for its own. Buy the group in one go and watch the whole bottom of the tank come alive.
Mistake two: gravel that files down their barbels
Those whiskers around a cory's mouth are tasting organs, used to sift the ground for food, and they are delicate. Coarse, sharp-edged gravel abrades them down to stubs, and worn barbels are often the first sign that the substrate is wrong — keepers sometimes blame infection when the real cause is the floor itself. Fine sand is the ideal: corys push their snouts into it and sift it through their gills in a way gravel never allows. Smooth, rounded fine gravel is an acceptable second choice. If your corys' barbels look shorter than they did in the shop, suspect the ground before anything else.
Mistake three: believing the cleanup-crew myth
Corydoras do not eat fish waste, and they are poor algae eaters. What they eat is what every other fish eats — they just need it delivered to the floor. A cory expected to survive on the crumbs that escape a school of tetras is a cory on a starvation diet, and it will thin out while the tank above it looks well fed. Drop sinking pellets, wafers or frozen foods where the group rests, ideally after the main feed so the faster fish are already full, and check at night that the food is actually reaching the bottom.
Mistake four: forgetting they live where the waste settles
Everything heavy in a tank ends up in the layer of water corys breathe and feed in. Ammonia and debris concentrate along the substrate, so the bottom-dwellers are the first fish to suffer when water changes slip. Weekly partial changes with a gentle siphon over the sand — hovering rather than digging, so the group is not buried in a storm of its own floor — keep their layer of the tank as clean as the surface. Pale, listless corys sitting apart from the group are telling you about the water before any test kit does.
A cory group asked to be a social animal, on soft ground, with dinner of its own and clean water at floor level, will repay you with constant gentle activity. Rotating the feeding, siphoning and water-change jobs through Care Teams on KinsLe means nobody in the household has to remember whose turn it is, and each finished task is marked with a timestamp the whole family can see.