Well i don't add them per sey i let them fall off the plants and decay; but you seem to be suggesting it is better to remove them from the water instead of letting them decay in the water and then adding external leaves.Because green leaves contain a big amount of nutrients and minimal amounts of humic substances. Fallen leaves that a tree has shed contain little nutrients because the tree has extracted them from the leaves and in turn lots of humic substances, which we want in the water.
Adding just dried leaves will not bring the positive effects of humic substances, while the additional nutrients will pollute the water. It's fine to FEED dried, green leaves to shrimp or the like, but just adding them without having consumers eating them, only ruins water quality.
I use the oak and in my case loquat leaves for cover more than anything, I use them because there stiffer than tank plant leaves, I dont need them for the water as mine [tap] is low PH and almost dead soft. Not sure if this is what your looking for but my thought on it.I've always been curious about this but why use external oak or maple leaves as oppose to just cutting a few leaves off your plants and letting them decay ?
Actually, we kinda need to add tannins/humic substances in these conditions as they help our fish retain the little minerals and salts they get, against the osmotic pressure. Almost forgot to mention that. Also they stabilize the pH somewhat in that low range. Doesn't take much. But most people achieve this using peat as it works faster and stronger than just leaf litter. TomC mentioned it here.I dont need them for the water as mine [tap] is low PH and almost dead soft.