Either fresh or dried, one leaf per 3-foot aquarium is sufficient for about a fortnight, so long as it does not begin to disintegrate or decompose. A leaf which begins spoiling should be removed at once.
The organic acids and tannins released by the leaves into water help reduce bacterial activity and are generally suitable for use even with delicate fish species.
They are apparently best suited to tropical fish like bettas and discus, and are popular with betta breeders in Thailand.
Reputedly, the leaves can also help encourage breeding.
Note that in addition to helping balance water PH, the tannins and other organic acids released can also cause the water colour to darken.
