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Don't forget to clean up all the food the Betta is not eating. It can quickly pollute a small aquarium. Chain store Bettas are not always in perfect health after spending weeks in a glorified shot glass. Although they can breathe air, they still need to use their gills, and they are affected by dirty water, which is usually full of toxic ammonia. If he started out in less than optimum health, he will need time to recover. A more secluded space could help. If you have access to a floating plant like water sprite or hornwort, it will help control toxins such as ammonia and nitrite. Some decorations or plants on the bottom of the aquarium will give him hiding places and make him feel more comfortable.
After the Betta recovers from his chain store odyssey, he will start taking food. Good luck, and please keep us posted. We like to help.