Indian Almond Leaf or naturally dried oak leaves work.It sours of tannins that may help . You need to rinse the leaves and crumble into the water to make it dark brown. You can also just place the leaf in the water but it takes a little longer for the tannins to seep out.
Tannins have antibacterial and antiparasitic properties and also help to acidify the water and turn it dark colored. This simulates a bettas natural habitat which can help put the betta at ease. The tannins will help to combat infection.