What a well-fed corydoras should look like
Corydoras are built low and rounded, and a healthy, well-fed fish shows it most clearly from the side and from above. Look for a smooth curve along the belly that peaks gently just behind the pectoral fins, where the stomach sits, and a back that stays broad and fleshy rather than pinched. Females naturally look rounder than males, especially when carrying eggs, so judge each fish against its own usual shape and against tankmates of the same sex where you can. Barbels matter too: a cory that is feeding well keeps full, intact whiskers, because it spends its day pushing them through the substrate after food. Shortened or eroded barbels alongside a thinning body suggest the fish is struggling at the bottom of the tank, not thriving.
Reading the droppings
Cory waste tells you both that food is going in and roughly how it is being processed. A well-fed corydoras produces short, dark strands or small compact pellets of waste, usually brown, green or the color of the sinking food it has been eating, and they break down quickly into the substrate. Droppings that turn long, stringy and pale, or white and trailing behind the fish as it swims, mean food is passing through poorly or barely at all, and are worth watching closely. Because corys feed on the bottom, you will spot their waste most easily on pale sand or in the front corners where the group rests. A sudden change across the whole group after a food swap usually points at the new food; a change in one fish only points at that fish.
The post-feeding belly check
The most reliable test happens in the ten minutes after food reaches the bottom. Drop the corys' sinking food in their usual patch and watch who actually eats: each fish should park over the food, work at it with quick mouth movements, and swim off with a visibly filled, rounded belly. If the same one or two fish always leave thin while bolder tankmates, or faster mid-water fish, clear the food first, the thin fish are being outcompeted rather than choosing not to eat. Feed a second smaller portion at the far end of the tank, or after lights-out when corys are naturally most active, and check whether the thin fish fill out over the following week. A belly that never rounds, even when food is placed directly in front of the fish, is a different problem from simple competition.
When to get a vet involved
One thin cory in an otherwise plump group deserves a week of targeted feeding and observation before alarm. Escalate sooner if the fish stops sifting the substrate altogether, if pale stringy waste appears alongside weight loss in several fish, or if a fish that was rounding nicely begins to hollow out despite eating. Those combinations move the question from feeding to health, and guessing at causes at home helps no one. Take your notes with you: when the change started, what the droppings looked like, which fish are affected and what changed in the tank beforehand. Keeping that dated feeding and body-shape record in your KinsLe Pet Passport means those observations travel with the fish's other records, so a vet sees the whole trend instead of a single worried snapshot.