Corydoras lay eggs individually on a surface, such as plant leaves, often the tank glass, wood, filter tube...just about any surface. As the eggs are "in the open" like this, they tend to get eaten fairly quickly, by any fish in the tank (not just the cories themselves). Over a couple of decades, I have had a very few fry hatch and survive; most of these I retrieved from the canister filter when I cleaned it. And this was a very heavily planted tank. I had many more pygmy cories hatch and survive because they were in their own tank with no other fish species, and more of the eggs survived.