Corydoras arrive stressed in a particular way: they are bottom fish who take their confidence from company, smooth ground under their barbels, and the ability to stay low and still until danger feels past. Acclimation that respects those three things — a gradual water changeover, the shoal kept intact, and a quiet release near the substrate — will have them nosing along the sand within hours instead of hiding motionless behind the filter.
What unsettles a corydoras most
Temperature and chemistry shifts matter, as with any fish, but handling and isolation hit this species harder than most. A single cory netted alone, held in bright light, and dropped into open water often stays frozen for a long time, while the same fish released with its companions settles in a cluster and begins feeding sooner. Their whisker-like barbels are delicate sense organs, so rough nets, sharp gravel edges, and being dragged across coarse substrate during release can damage the very tools they use to find food.
Preparing the bottom before the bag opens
Check that the tank floor offers a patch of smooth sand or fine, rounded substrate where the newcomers can land, with a cave, plant thicket, or piece of driftwood close by. Test the water and confirm it is stable, then lower the lights and keep faces and tapping fingers away from the glass. If your resident fish include boisterous feeders, feed them lightly at the far end shortly before the release, so the corydoras are not barged aside while they are still disoriented.
Floating and mixing for a bottom fish
Float the sealed bag until the temperatures match, then open it, roll the top down so it floats, and add a small cup of tank water at intervals, letting the fish adjust in stages. Corydoras occasionally dart to the surface for a gulp of air, which is normal behaviour for the species and not by itself a sign of trouble. Once the bag holds mostly tank water, tilt it gently in the water or lift the fish in a soft, fine net, keeping each trip short and low so nobody is stranded flapping in dry air.
Release low, close to cover
Lower the corydoras in near the sand rather than letting them fall through the full depth of the tank, and aim the release beside shelter so the shoal can shuffle straight under a leaf or ledge together. Discard the bag water down the drain, never into the aquarium. For the first hours, expect clustering, stillness, and short exploratory shuffles; an occasional dash to the surface remains normal. Offer a small sinking food after they have had time to settle, placed where they are resting rather than scattered for faster fish to steal.
Reading the shoal over the next days
Healthy settling looks like barbels sweeping the sand, fins held naturally, and the group spreading out a little more each day. Frayed barbels, rapid breathing that does not ease, or a fish lying tilted and separate from the group deserves a prompt water test and, if it continues, advice from an aquatic veterinarian, since bottom fish show trouble late. Recording the arrival, the water readings, and the first feeding in KinsLe Family Sharing keeps every family member tending the tank aligned on how the corydoras are progressing.