Unless you just have a very grumopy territorial betta, you've got alot of choices. Go for small peaceful fish that aren't fin nippers and don't get anything like guppies that your betta might think is another betta. I have bettas (one in each of two tanks!) with cory cats, neon tetras, glowlight tetras, ottos, zebra danios, white cloud minnows, harlequin rasboras, rummy nose tetras, flame tetras and dwarf gouramis. No problems. Just be prepared to watch them initially so if there is a problem, you can seperate them.