Bottom fish need a bottom designed for them
Corydoras spend their whole lives on the substrate, sifting it mouthful by mouthful through their barbels - the whiskers they taste and feel their food with. That one fact designs the tank. The floor should be soft, fine sand, deep enough to push around, because sharp gravel abrades barbels down to infected stubs and rules out the sifting behaviour that makes these fish so entertaining. A 20 gallon long tank, with its generous footprint, suits a group of most common species such as bronze, peppered or panda corydoras better than a tall tank of the same volume. Leave much of the floor open; a cory tank is furnished around the edges, not across the middle.
Six of one species, not two of three
Corydoras are genuinely social, and a lone cory is a stressed cory that hides and fades. Buy at least six, and make them the same species: mixed oddments of bronze, panda and julii do not properly shoal together, while six of a kind will rest in a row, forage as a group and occasionally shoot to the surface together for a gulp of air - a normal cory habit, not a sign of trouble. Add the group together once the tank is cycled, and choose plump, active fish with full barbels from the shop tank, since barbel damage there tells you how they have been kept.
Water and filtration for floor-dwellers
Most tank-bred corydoras are content in ordinary community water around 72 to 78 degrees - some species, like the peppered cory, prefer the cooler end, so check your species rather than assuming. Keep flow gentle to moderate and the filter intake guarded, and be diligent about the bottom's hygiene: waste settles where corydoras live, and dirty substrate is the usual road to barbel erosion and bacterial trouble. A weekly gravel-vacuum of open sand areas - hovering the siphon just above the surface so sand stays put - plus partial water changes keeps the floor as clean as the fish need it.
Feed the bottom on purpose
The most common way beginners starve corydoras is by believing they are a clean-up crew that lives on leftovers. They do not. Feed them directly with sinking wafers, small pellets or crumbled food that reaches the floor, ideally just after the main feeding or at lights-out so mid-water fish do not intercept everything, and offer frozen bloodworm or daphnia as a regular treat - a group of corydoras descending on bloodworm is one of the joys of the hobby. Watch bellies: a healthy cory is rounded when viewed from above, and in a group you can see at a glance if one fish is falling behind.
Cover, companions and settling in
Give the group somewhere to be shy: a driftwood tangle, a cave, or a cluster of plants at the back where they can rest out of the light during the day. Floating plants or a dimmed corner helps new fish settle, since corydoras come from shaded streams and bold lighting makes them hug cover. Tankmates should be peaceful mid- and top-water fish that leave the floor alone; avoid anything that nips barbels or competes aggressively on the bottom. Expect a few quiet days after introduction, then increasing expeditions across the open sand as the group gains confidence.
Feeding the floor is a separate job from feeding the tank, and it is the job most easily forgotten when several people share fishkeeping: KinsLe's Care Teams lets you assign bottom-feeding and the weekly floor clean to a named person, with finished tasks ticked off and timestamped for everyone to see.