Garlic smells (tastes?) wonderful to betta fishies. You put it on their food. Not the whole clove, of course, just the juices: Crush a fresh garlic clove to release the juices, then put the juice on her food. How you do this is up to you, but it doesn't take much. I use a spoon, crush the garlic with the back of it, then press the back of the spoon against a few food pellets and roll them around. My fishies love the stuff. I've got 2 picky eaters and they both gobble the food right up.
I've heard you can use the garlic oil from capsules too, but haven't tried that.