I heard that they can actually smell the food, and they seek it out. They can find it when its dark, I've fed some when it was dark and they found it no problem.
Many predatory fish are nocturnal, using their very keen sense of smell to find their food. Corys use their barbells to sniff out food (don't know if sniff is the right verb). Most fish have great eyesight, not sure how crisp it is, but they can definitely detect movement (ie. a piece of food falling to the bottom).