Start with the barbels — they report on the whole tank
The whiskers around a corydoras mouth are sensory tools it pushes into the substrate all day searching for food. On smooth sand in a clean tank they stay long and fine. On sharp gravel, or on any bed where waste is rotting underneath, they wear down, inflame and can erode to stubs, opening the door to bacterial infection around the mouth. Barbel erosion is therefore rarely just a fish problem; it is the tank telling you its floor is wrong, and the fish with the worst whiskers is simply the messenger.
Breathing and the surface: know the normal version
Corydoras can swallow air at the surface and absorb oxygen through the gut, so an occasional quick dash up is ordinary behaviour. What is not ordinary is the whole shoal doing it constantly, a fish hovering near the surface, or gills pumping hard while the fish sits on the bottom. That pattern points at oxygen shortage or ammonia in the water, and corydoras resting in the mulm at the bottom are the first to suffer when the bed or filter is neglected. Alongside breathing changes, watch for fins clamped or fraying, white grains across the body, and flashing — a fish flicking itself against the sand as if its skin itches.
The loner is a symptom, not a personality
A healthy corydoras shoals; it rests in a loose huddle and feeds in company. One fish parked alone under a plant, refusing to join the group at feeding, is one of the earliest and most reliable illness signs in this family, often appearing before any mark shows on the body. Colour washing out to a dull grey, a belly that sinks in, or a fish that lists slightly as it rests all deserve the same response: test the water that day and watch the whole group, because corydoras usually share whatever one of them has.
Treatment traps specific to corydoras
Corydoras are armoured rather than scaled, and they cope badly with salt and with several common medications at full strength. A remedy that is safe for the tetras above them can burn a cory's skin or stress it further, so check cory safety specifically before anything goes in the water, and treat a bare hospital tank as a last resort for a fish that depends on its shoal. Prevention carries most of the weight: smooth sand or rounded fine gravel, a siphoned bed, and a quarantine period for new fish before they join the group.
The point at which to bring in a vet
Seek an aquatic vet if barbels are eroding despite a clean, smooth substrate, if several fish show rapid breathing or white spot together, or if a wasting fish keeps losing condition while eating. Milky patches spreading on the body or a fish unable to stay upright also warrant prompt advice rather than home experiments. Photographing individual fish weekly and noting the shoal count, breathing and feeding behaviour in your KinsLe Pet Passport turns a vague 'one looks off' into a timeline a vet can actually use.