Indian almond (or ketapang) is a tree that grows everywhere in Thailand, the betta's natural habitat. The leaves release tannins into the water, which colours it slightly brown and gives it a distinctive tannin smell (which I quite like). The tannins are beneficial to the fish and seem to keep them healthier. They strengthen the fins and slime coat, making the fish less prone to illnesses like finrot. They might also stimulate breeding (if that's what you're trying for.)