Oak leaves or any other brown leaves are added to tank primarily for CRS. They love the biofilm and it's good food for newly hatched CRS. Typically, what I do, is let the leaf sit in a matured tank for a few days, so that biofilm would develop, it'll be slightly slimy to the touch after a few days. When it's ready, I transplant the leaf to the shrimp tank, they'll go nuts for the biofilm.