Feeding Bettas

Bettas have upturned mouths and are primarily carnivorous surface feeders. In the wild, bettas 'feed on zoo plankton and the larvae of mosquitoes and other insects, such as flies, crickets, or grasshoppers.
Bettas which feed on wide range of foods live longer, have richer colors, and heal fin damage more quickly.

Typically, Betta pellets are a combination of mashed shrimp meal, fish meal, brine shrimp, bloodworms, and vitamins. Bettas also will eat live or frozen bloodworms, brine shrimp or daphnia.

For variety and fiber, bettas are fed finely-chopped, high-protein vegetables, such as soybeans, green beans, broccoli, corn, and carrots. Some bettas subsist on dried flaked food suitable for tropical fish, because although this feed reduces their coloring, the bettas are able to digest this better than pellets.

However, feeding bettas vegetables only is not a good idea since they are carnivorous and do better with meat products. Bettas can get constipated when their diet lacks variety.
If their stomach looks swollen, feed them food with fiber.