Readiness shows in the barbels and the foraging
A young corydoras catfish is ready for the main tank when it spends its time doing what adults do: nosing through the substrate in short bursts, resting with siblings, then foraging again. The whisker-like barbels around the mouth should be full, even, and unshortened, because a young corydoras relies on them to find food in sand or fine gravel, and damaged barbels in a youngster often signal rough substrate or poor water in the grow-out tank. Size matters mainly against the main tank's biggest mouth — young corydoras are safe from adult corydoras themselves, but a large angelfish, gourami, or other predator in the main tank could take a small one. When a youngster is stout-bodied and clearly beyond bite size for every resident, it passes the size test.
Move the shoal, not a single youngster
Young corydoras catfish are social from the start and a lone youngster in a new tank often hides, skips meals, and fades. Move at least a small group together — ideally enough that, combined with the adults, the corydoras shoal numbers six or more — so the youngsters can copy adult behaviour and learn where food lands. Watch how the group behaves in the grow-out tank beforehand: youngsters that already rest in a loose pile and forage side by side will transfer that confidence to the main tank. If one young corydoras is much smaller or thinner than its siblings, leave that one behind to grow on rather than holding the whole group back, then add it later once it catches up.
Preparing the main tank floor for young corydoras
Before the move, check the main tank from a young corydoras catfish's point of view: a soft sand area or smooth fine gravel for barbel-safe digging, low cover from plants or small caves at floor level, and no sharp-edged decor at the bottom. Match temperature and water chemistry between tanks and acclimate slowly, since young corydoras react badly to sudden changes even though adults tolerate a wide range. Release the youngsters near the substrate in a planted corner rather than pouring them in at the surface. Scatter the first feed along the bottom after release — sinking micro foods that reach the floor matter because mid-water fish in the main tank will intercept anything that drifts down slowly.
Settling checks for the first several days
A settled young corydoras catfish forages in the open, rests calmly with the group, and shows a rounded belly after feeding. Warning signs are specific to this fish: a youngster sitting alone under a leaf all day, barbels looking shorter or reddened, rapid gill movement, or repeated darting to the surface far beyond the occasional gulp of air that is normal corydoras behaviour. If several youngsters look unsettled, test the main tank water before assuming the move itself failed, because ammonia near the substrate hits bottom-dwellers first. Keep notes on which youngsters joined, their sizes, and any barbel concerns in KinsLe through Care Teams so the next water change or vet conversation starts from accurate, dated observations.