Depends on the betta, and the size of a tank. I have kept bettas in larger aquariums, 10 or 15 gallons with small corydoras catfish (some species get larger than others) and ottos, and in one case neon tetras. Corries or ottos are pretty safe in any case, as long as the betta isn't super agressive like Serpent is, but tetras can cause problems and you have to be willing to seperate them if the betta tries to eat them, or if the tetras nip at the betta's fins, and many of them do. They wouldn't have to be neons, any small peaceful tetra can be attempted, except serpae tetras. Those things are almost always very nippy.