Corydoras Are a Group Fish First
Every corydoras number starts from the same fact: these are schooling fish, hardwired to feed, rest and travel in company. A lone corydoras, or a pair, spends its days stressed and mostly hidden, and the charming behaviour people buy them for, the bustle of a group snuffling through the substrate together, only appears at six or more. So the question in a 10-gallon is never whether one or two large corydoras fit. It is whether a proper group of six fits, and in ten gallons that group has to be made of the smallest members of the family.
The Dwarf Species That Fit
Three species carry the 10-gallon plan. Pygmy corydoras are the smallest, hovering around an inch as adults, and unusual in the family for swimming in mid-water as a loose shoal as well as working the bottom. Habrosus corydoras, sometimes sold as salt-and-pepper corys, stay similarly small and stay closer to the substrate. Hastatus corydoras are the third dwarf, less often seen in shops. Six of any one of these, in a planted 10-gallon with a gentle filter, is a stocking the water can genuinely support. Choose one species and buy the six together; a mixed trio of two species gives neither group its school.
Why the Familiar Species Do Not Fit
The corydoras in most shop tanks, bronze, albino and peppered, are not small fish waiting to be counted differently. Adults reach two and a half to three inches, deep-bodied and active, and six of them in a 10-gallon produce a waste load and a demand for floor space the tank cannot meet. They do not stay small because the tank is small; they grow, crowd, and suffer the water quality that follows. This is the most common corydoras mistake in the hobby, bought as inch-long juveniles with the best intentions. If those are the species you want, and they are wonderful fish, the honest answer is that they are a 20-gallon group, not a 10-gallon one.
Floor Space Matters More Than the Gallon Count
Corydoras live on the bottom, so the resource they consume is footprint. A standard 10-gallon gives them a modest rectangle of it, and how you furnish it decides how much is usable. Fine sand is the kindest substrate, letting them sift without wearing down the delicate barbels they taste with; sharp gravel can erode those barbels and invite infection. A piece of smooth wood, a small cave and a patch of leaf litter give the group resting cover, while leaving an open sandy area at the front where the whole school will gather to feed and where you can count all six daily. A group you can count is a group whose losses you will notice in time to act.
When to Size Up Instead
Choose the dwarf school happily if small fish delight you. But if your heart is set on the bronze and peppered fish in the shop, or on watching a dozen corydoras pour across the sand at feeding time, the right move is to let this question change your tank rather than shrink your group: a 20-gallon long, with its wide footprint, is where the common species and a group of eight to ten come into their own. Matching species to the glass in front of you, rather than the one you wish stood there, is the whole art here; for sorting the dwarf from the merely young, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.