Otocinclus eat two common kinds of algae, brown diatom and soft green algae. Some otos will even eat black beard algae.
However, otos love to eat algae so much that they will clean the entire tank of it. Because there isn’t enough algae to sustain them, you’ll need to feed them other food, too.
Supplement their diet with algae wafers and sinking pellets. Otos won’t eat other plants in the tank, but they would appreciate fresh vegetables.
You don’t need to provide them with extra food every day, just feed them two to four times a week.