A few people near us own magnolia trees, and I regularly collect the dry leaves and boil them, then add them to my vivarium for the frogs.
I'd like to do something like this with my aquarium, just add a bunch of them to the tank for the cories to mess around in. Will they release a lot of tannins? I don't mind if they release a little bit, but I don't want to add more to what the wood is causing, because then I wouldn't see my fish.