From The Aquarium Wiki
Vegetable food consists of meals presented to fish with various levels of concentrations of plant matter.
Since many fish are primarily or exclusively vegetarian, typical foods do not provide the nutrition they require, usually being based on animal protein sources.
Commercial products range from flake food with a green lid that say "veggie" all over the can - one of which, for example, has as its first four ingredients salmon, cod, halibut, and herring, before listing kelp and spirulina, to flakes that are more prominently plant-based (such as Spirulina 20), to frozen plant-only products.
Many aquarists meet their herbivorous pets' needs using a variety of techniques. One is to try to promote algae growth in the aquarium, which many such fish will happily browse upon. Another is to experiment with various land plants, whether fresh or frozen, to see which ones their fish will accept and ideally, thrive upon. Common suggestions for this purpose are spinach, lettuce, peas, and zucchini. The key when experimenting is to provide the food regularly, in hopes that the fish will "learn" to accept it, but in very small quantities, so as not to add too much rotting vegetation to the tank.
Another form of vegetable food, often used by breeders, is so-called "green water". This is water in which microscopic flora have been grown in a separate tank, in order to provide a live food appropriate for very small fish.