A bottom dweller's fireworks night
Most fish meet fireworks as light. Corydoras meet them as ground movement. These little armored catfish spend their lives pressed to the substrate, sifting sand through their barbels, and their bodies are tuned to vibration arriving through the bottom of the tank — which is precisely the route bangs take, up through the floor, the stand, and the glass base. A cory school under stress does two things: it freezes in a huddle, or it erupts — the whole group shooting upward and around the tank in a blind second. That eruption is the risky moment, when fish meet glass, filter intakes, and sharp decor at speed. The other thing to know is reassuring: a single cory darting to the surface for a gulp of air is ordinary behavior on the calmest night of the year, because these fish supplement their breathing at the surface. Panic is a group event; a solo dash is just a breath.
Shelter at sand level, arranged early
Do the work in the afternoon, at the level the fish actually live. The group needs at least one low, roofed space big enough for all of them at once — a cave, a piece of wood forming an overhang, a coconut shell, or a dense thicket of plants reaching the sand — because corydoras cope socially, shoulder to shoulder, and cover that fits only two of six fish leaves four exposed. Check the path to that shelter is across open sand rather than over coarse gravel; frightened cories scramble, and rough substrate is hard on barbels and bellies. Look along the tank's glass line at their level for anything topple-able — a loosely stacked stone is a hazard when six fish hit the area together — and settle or remove it. Feed the group in the late afternoon, sinking food onto the sand where they expect it, and let the evening's feeding be finished before the noise starts.
While the bangs are going
Dim and steady wins the night: a low room light left on, no tank-light switching, no tapping to summon them out for a look. A cory school sitting motionless in its cave for two hours is not a problem to fix — stillness is the strategy, and fish that feel hidden endure vibration remarkably well. What you should not do is intervene physically. Netting a darting cory mid-panic multiplies the panic and risks real injury against the glass; if one fish ends up out of the water somehow, wet hands and a quick, gentle return are the whole remedy. Keep the stand's surroundings calm — speakers, dancing feet, and slamming doors all feed the same vibration channel the fish are reading. If the tank stands on a hard floor in a room facing the display, you cannot lift a stocked tank to cushion it mid-evening, but folded blankets or a thick rug pressed against the stand's base do dull the worst of the vibration arriving through the floor.
Reading the group the morning after
By morning, a cory group that coped will already be out sifting the sand in its loose morning patrol, and breakfast behavior is the cleanest verdict: cories that come to a sinking wafer promptly had an ordinary night. Check barbels and bellies on the fish you can see clearly — whiskers should be full-length and even — and watch for any fish still isolated and motionless away from the group, which earns it a quiet day and close watching rather than handling. A gentle water change suits the morning after, since stress nights always leave the water slightly worse than they found it. Uncertainty about anything you noticed can be settled later, once the group is feeding normally again. Put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.