As I told to one of my friends
First identify species, then fed-This diagram will help in the classification, since some apple snails prefer aquarium plants,other strive for vegetables, and others [if not all] simply are scavengers that will clean glass and gravel of algae. If I remember well, green algae is their favorite.[in the case of my diffusa] In my opinion, its really hard to underfeed an apple snail. This critter will always find something to eat at any tank, provided you make food go into your tank.