A corydoras shoal is quietly well equipped for a power cut, because these catfish supplement their gills by gulping air at the surface. Your job during a long outage is to protect that surface access, hold the temperature steady, and avoid stirring a resting shoal's bottom world into water that no running filter can clean again.
Guard the surface they breathe from
Corydoras in a still tank will make more of their characteristic darts to the surface, and that behaviour is normal coping, not distress by itself. Keep the water level a little below the rim and the cover loose enough that fresh air sits above the surface — a tank sealed completely tight for warmth defeats the very route these fish use to breathe. Do not add extra fish, move the shoal to a small container, or net them for inspection; a corydoras resting group in its own tank is using less oxygen than a chased one. If a battery air pump is available, a gentle airstone adds a margin, set low so the bottom stays calm.
Hold warmth around a bottom-dwelling shoal
Wrap the corydoras tank in blankets or duvets, including over the top, because the large water volume cools slowly if it is not leaking heat through bare glass. Corydoras live at the bottom, the last layer to change temperature, which buys them time in a short cut. What harms them is a sudden correction: no jugs of warm water poured in, and no heater run in bursts off a generator without a thermometer check, because a shoal huddled at the glass can be scalded or shocked by a local hot plume. Warming the room gently, if you safely can, raises the whole tank evenly instead.
Leave the substrate and the feeding alone
Resist cleaning during a corydoras power cut, however overdue it feels. The substrate in a cory tank holds both waste and the snails-and-crumbs economy the shoal picks through, and raking it releases a cloud of debris into unfiltered water. Feeding should stop or shrink to a token amount: sinking wafers that would normally be found and eaten within the hour sit and soften in still water, and a resting shoal's appetite drops with the temperature anyway. The beneficial filter bacteria survive a cut of many hours in the damp filter; keep the media wet in tank water rather than rinsing it, so filtration resumes properly when power returns.
Read the shoal before you intervene
Occasional surface trips are the corydoras system working; constant frantic dashing, the whole shoal hovering off the bottom, or individuals rolling or listing are the signs that oxygen or water quality has genuinely failed. At that point a partial water change with conditioned water matched closely in temperature, poured gently at the surface, is worth its disturbance. When the power returns, expect the shoal to resume bottom-sifting gradually, and watch barbels and fins over the next days for any soreness. Through a long cut, KinsLe Kinny can be asked which sign matters next, with the shoal's details kept alongside its profile rather than in your memory.