Clown loach, by a country mile i'd say, are the big one here. They migrate in huge numbers to spawn, a feat totally impossible to create in an aquarium, and when they do they are one of the most prolific spawners out there, creating millions upon millions of fry. They get netted out in huge quantities and it's these very young and skinny, often iIl looking fish, that you see in stores.
Some specialist fish farms have cracked it, but only be using hormones, but i'm afraid for the rest of us it is virtually impossible.
That is why clown loach are so damn expensive. They say there's not much money in breeding fish but let me tell you, any casual hobbyist who cracks the clown loach code, successfully and regularly, will get pretty rich, and quick.