What a Big School Does Differently
Somewhere past ten fish, a corydoras group changes character. A group at this size does not merely forage; it flows, pouring across the sand in a loose body, splitting around the wood and rejoining, resting in a drift of fish several layers deep. Individual shyness disappears, because in a crowd every fish is anonymous, and you will see daytime activity that a minimal group reserves for dusk. A 40-gallon is the first tank in the common range with the floor and the water volume to hold that crowd of adult fish indefinitely, which makes it the size where corydoras stop being a supporting act and become the reason for the tank.
One Large School or Two Honest Groups
Forty gallons offers a choice the smaller tanks do not. One species at twelve to fifteen gives you the full spectacle of a single school behaving as one organism, and for most keepers it is the better show. Two species can work, but only under the discipline that governs all corydoras keeping: each species gets a real group, at least six and better seven or eight, because three-and-three in a bigger tank is the same mistake as in a small one, just with more water around it. If you mix, choose species that differ clearly in pattern, such as bronze with pandas, so you can read each group's numbers at a glance during your daily count.
The Breeder Footprint Versus the Tall Tank
Not all 40-gallons are equal for a bottom fish. The breeder model trades height for a long, wide floor, and for corydoras that trade is pure profit: the school's whole life happens in the bottom few inches, and the breeder gives them a floor closer to a playing field than a rug. A tall 40-gallon holds the same water in a footprint nearer a 20-gallon's, and the school will feel that. If the tank is tall, stock it as a floor-space tank, twelve of a smaller species rather than fifteen of the largest, and furnish the bottom densely. When buying for corydoras, bring a tape measure for the footprint and let the gallons look after themselves.
Feeding a Fifteen-Fish Bottom Crew
A big school eats like one, and feeding becomes the main management task. Sinking wafers and pellets should be spread across two or three spots on the sand so the boldest fish cannot monopolise a single pile, and the amount should be judged by the school finishing it within a few minutes rather than by the packet's optimism. In a community tank, feed the bottom crew after the main feeding, or at lights-out, so food reaches the floor past the tetras. Watch the smallest two or three fish weekly; in a large group the runts are easy to overlook, and a fish losing condition among fourteen healthy ones is telling you about either its own health or its place in the queue.
Breeding Behaviour in a Big Group
A settled school in 40 gallons is the likeliest of all setups to show you spawning: after a larger, slightly cooler water change, males pursue a female in the classic T-position dance, and eggs appear stuck one by one to the glass, plant leaves and filter housing. In a community tank most eggs and fry will be eaten, and that is a natural outcome rather than a failure. If raising them tempts you, eggs can be rolled gently off the glass into a small rearing tank. Either way, spawning is the school's own verdict that the numbers, the floor and the feeding are right. A group this size deserves deliberate planning before the first bag is floated; put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.