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If you get new fish, be sure to add them to the tank before the betta ever sees it. Most bettas will see new fish as invaders in their territory, but if the fish are already there, they'll just be seen as furniture. I understand that cories do much better in groups of four, so you might get more of those. Avoid any fish with fancy fins, like guppies. A pair of mollies or another livebearer is usually a good choice for bettas. Small fish like neon tetras tend to nip at their fins, so avoid those.