From above is the corydoras view that works
A corydoras catfish shows its sex most clearly to an eye looking straight down. As a female matures she widens through the middle of the body, her outline swelling gently behind the head so she looks round and full from above. A male of the same age and feeding stays narrower, his sides running in a sleeker, more even taper to the tail. Side-on, the armoured plates and busy whiskers hide that difference almost completely, which is why so many keepers stare at the side of the tank and conclude the fish are identical.
Why youngsters need weeks, not a glance
A young corydoras bought at small size has not yet put on the adult body that carries these clues. Females fill out as they approach maturity and begin producing eggs, so the roundness that gives them away is partly a sign of readiness, not just sex. Comparing one rounded fish against slimmer tank mates of the same species and purchase batch is far more reliable than measuring any single fish. Feed the group evenly, photograph them from above at intervals, and let several weeks of growth draw the contrast the shop tank could not show you.
Secondary clues in fins and behaviour
A mature male corydoras may show slightly longer, more pointed fins, and during courtship he becomes the pursuer, shivering alongside a female and nudging at her head in the classic pre-spawning dance. A female carrying eggs looks deep-bodied even from the side, and if spawning follows, she is the fish seen carrying a small cluster of eggs held between her pelvic fins toward a chosen surface. In a young, non-breeding group none of this may appear, so treat fin shape as a supporting hint and the top-down body width as your main test.
Mistakes that mis-sex a young cory
The commonest corydoras error is calling the greediest fish female, because a well-fed juvenile of either sex looks round for an hour after a good meal. Judge shape in the morning before feeding, when stomachs are empty and the true outline shows. Comparing across species misleads too, since some corydoras are naturally chunkier than others. Albinos and long-finned varieties follow the same top-down rule, though their colouring hides subtler fin clues. And a single fish kept alone gives you nothing to compare against, which is another reason these social catfish belong in groups anyway. Photographs help here as well: a phone picture taken from directly above, with the group resting over the same patch of sand each time, removes the guesswork that remembered shapes invite.
Why the answer is worth recording
Sexing a corydoras group properly pays off when you plan the tank's future. A group skewed heavily to one sex behaves differently from a balanced shoal, and anyone hoping for the small delight of cory eggs on the glass needs to know a mature female is actually present. Once your top-down comparisons settle which fish is which, the answer should not live in your memory alone. Entering each confirmed corydoras into its KinsLe Pet Passport keeps its sex alongside its other records in one portable packet, ready whenever the group moves tanks or changes hands.