Six Is the Floor, Not the Target
Corydoras keeping has a minimum, six, below which the fish are surviving rather than thriving, and 20 gallons is the first common tank size that lets you stock comfortably above it. Eight to ten fish gives the school what six only promises: enough company that the group splits and regroups naturally, some fish resting under the wood while others forage, the whole tank's bottom in gentle motion at feeding time. It also gives you margin, because a school of nine that loses a fish is still a school, while a school of six that loses two is a problem you must shop your way out of quickly.
Which Species Suit a 20-Gallon
This is the size where the classic corydoras come into play. Bronze and albino corydoras are the hardiest and the largest of the common choices, reaching around three inches, and a group of eight fills the tank handsomely. Peppered corydoras run slightly smaller and tolerate cooler water than most tropical fish, which suits unheated rooms. Panda corydoras are the most delicate of the three and reward stable, mature water. The long 20-gallon, with its wide low footprint, serves all of them better than the tall model, because every extra inch of floor is territory this bottom-dwelling family actually uses. Match the species to your water and your room, then commit to that one species for the whole school.
One Species, One School
The temptation in a shop, faced with four attractive corydoras species, is three of each. Resist it. Corydoras recognise their own kind by pattern and behaviour, and a trio is below the comfort threshold for each species at once, giving you four nervous trios instead of one confident school. A single species in full numbers rests together in a visible pile, feeds as a unit, and in a mature tank may reward you with eggs stuck to the glass on a rainy-feeling morning after a cool water change. If a second bottom species is irresistible, it should be a genuinely different animal, such as a small pleco, that does not compete for the school's role.
Laying Out the Floor for a Bigger Group
Eight to ten adult corydoras need the bottom arranged for a crowd. Give them a broad open sand area at the front for group feeding, since scattering food across sand rather than dropping it in one pile lets the shyest fish eat without being shouldered aside. Behind it, layer cover in depth: wood, smooth stones, plant thickets, so the group can spread through several resting spots instead of stacking into one cave. Fine sand remains the substrate of choice, protecting the barbels they feed with. Feed the school deliberately with sinking food of its own rather than assuming leftovers will reach the bottom; in a stocked community tank, they often do not.
Tankmates That Leave the Bottom to Them
The ideal companions for a 20-gallon corydoras school live above it: a school of small tetras or rasboras in the middle water, perhaps a honey gourami or a betta with a proven calm temperament near the top. What to avoid is competition on the floor itself. Other bottom fish of similar habits, such as loaches in numbers or several plecos, turn the substrate into contested ground and the corydoras school into the losers at every feeding. Keep the bottom a one-family neighbourhood and the school will repay you by being visible all day instead of only at dusk. Weighing bronze against panda for your water deserves care before you commit to a school; put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.