I think you will find they lower the pH, due to the tannins being acidic, but I cant see them absorbing calcium from the water, so hardness shouldn't really be affected.
Out of interest the aquarium remedies Melafix and Pimafix are made from oils extracted from the Cajeput and Bay tree leaves respectively.
So the idea of using leaves as an anti-biotic is not a crazy one. The Indian Almond leaves just bypass the stage where it gets distilled out of the leaves and put in a little plastic bottle.
I have 4 massive oaks right out my front door, I want to put some oak leaves in a couple of my tanks, is there anything special i need to do to them before they go in my tank?