There are safe leaves, and there are dangerously toxic leaves. Generally, hardwood tree leaves (oak, maple, beech) are safe as far as the leaf itself is concerned. I use oak leaves. Collect them when they are dead dry, as in autumn when they fall from the tree. Make sure none have bird droppings. I sometimes rinse them for dust, that's it. Boiling will cause them to decompose much faster, and you also lose the benefits from the decomposition process. Leaves will produce infusoria and all fish will graze them. Excellent fry food.
Make sure you collect them from a safe area...no chemicals, pesticides, fertilizers, industry, roads, etc. I had an oak tree in my back yard until I moved, and I knew those leaves were safe as I used none of these products.
Being organic, dried leaves will possibly lower the pH, this depends upon the level in the source water. They should not contribute to nitrates. I have left mine in some tanks until they were skeletal and even beyond, in others I replace them as they begin to fall apart to keep a layer of "leaves" on the sand.
I posted a link to an excellent PFK article on dried leaves, you might be able to find it, about a month or so ago.
