Corydoras are technically omnivores but they require a high amount of protein. This means that they prefer a meaty diet.
In the home aquarium, our bottom-feeding friends will eat whatever little critters they can find in the substrate and on biofilm growing on plants and wood, along with uneaten food that falls to the substrate from fishes who miss it in the mid- and upper water. Corydoras should be fed specifically however, and should not be made to rely on the ‘leftovers’ that falls to the substrate.
Corydoras relish insects and worms. Foods with these ingredients are available in most big box pet stores and local fish stores. Make sure to examine the ingredients on the label and choose foods where the first few ingredients listed are insects/worms/fish. Avoid first ingredients that are soy/wheat/grains as they are mostly fillers and are not nutritious. Algae or veggie wafers are ok for a treat, but are not very useful or nutritious for the fish as well.
Click the link for your country below, as there are different foods available in different countries: