Corydoras sizing starts from a group, not a fish. A single cory in a tank is a stressed cory — these are shoaling catfish that rest touching each other, forage in loose formation, and visibly relax in company. So the minimum tank is the one that houses six of them properly: a twenty-gallon, and preferably the long format, giving the school a foraging floor rather than a deep water column they barely use.
Footprint over height, sand over gravel
Corydoras live on the bottom and taste their way through it, sifting mouthfuls in search of food. A twenty-gallon long offers several square feet of that floor; a tall tank of the same volume offers far less and wastes the difference. Substrate is the second half of the housing decision: they push their sensitive barbels into the ground constantly, and coarse or sharp gravel wears those barbels down, opening the door to infection. Sand, or fine rounded gravel, with a smooth open patch kept clear for feeding, is the setup their anatomy asks for.
Species change the maths
The corydoras label covers fish of very different adult sizes. Dwarf species like pygmy corys can keep their six-fish shoal in the twenty-gallon comfortably and even share it widely. The popular bronze, peppered and sterbai corys reach around two and a half to three inches and fit the twenty-gallon floor as a group of six. The biggest species, and any school you hope to grow to ten or twelve — which is where their shoaling behaviour truly shows — belong in thirty gallons and beyond, with the floor space to match.
Why the group size is not negotiable
Keep three corys in a big tank and you still get shy fish that hide behind the filter and dash out only at feeding. The shoal is the security system: more bodies means more eyes on watch, and individuals spend the day in the open doing the busy, snuffling work people buy corys for. Budget and size the tank for six from the start, add them together so the group forms at once, and expect a bolder, more active bottom layer than the same money spent on three ever buys.
Tankmates and feeding at floor level
Corys are among the most peaceful fish in the hobby and pair with almost any community fish that will not nip or outcompete them. Their one vulnerability is the dinner table: mid-water fish finish floating food before a scrap reaches the bottom. Feed sinking wafers or pellets after the main feed, a couple of times a week drop food in after lights-out, and watch that bellies stay rounded — a cory school thinning out in a well-fed tank is being outcompeted, not undersized.
The dash to the surface is normal
New keepers panic the first time a cory shoots to the surface, gulps air and dives again — it looks like gasping, but corydoras breathe partly through the gut and top up this way even in well-aerated water. Occasional darts are a species trait, not a symptom. What does deserve a prompt water test is the whole school doing it constantly, or individuals hanging at the surface listlessly: then oxygen or ammonia really is the problem, and the fish on the floor of the tank are your earliest messengers that its balance has slipped.
Write the school's basics into their record — tank size, substrate, group count and feeding routine — on their KinsLe Pet Passport, and the setup that keeps those barbels healthy travels with the fish if they are ever rehomed or handed to a sitter.