First thing you need to know is don't overfeed the fish. You should feed adult betta fish just once a day and for the babies twice a day. Once you've fed them don't just leave them. Spend your time for a moment to see your betta eat. A betta fish should take two to five minutes to make itself full.

The most common food for betta fish is water flea. Click here to see how to breed water flea. Betta fish prefer live food, but feeding them freeze dried food is a more convenient and affordable alternative. Never use worms caught from the garden or caught in the wild for your betta fish as they may have been subjected to pesticides.

Now here's some useful tips for you (thanks to aquariumguys.com for this tips):

• Just like most fish, betta fish can go a couple days without eating, so don't worry about leaving for the weekend. But if you leave for a longer period of time, be sure to have someone feed the fish (or use a time-release food block, or automatic feeder) and clean their water (if smaller than 5 gallons it will need cleaning at least once a week).
• It is good practice to skip a day of feeding once a week, as it gives the betta fish's digestive system time to recover and clean itself out.
• IMPORTANT: While betta fish will stop eating dried or frozen food once full, they will continue eating as long as there is live food in their aquarium. Thus it is extremely important not to overfeed them with live food, as they will literally eat themselves to death.
• If your betta fish develops a paunch, or an overpronounced belly, you may be overfeeding it. Conversely, if he appears gaunt or thin, he may have an infection or need more food.