I just yesterday picked up some more leaves from large birch tree to replace those I have in driftwood tank with five assorted pleco's.
The ones in the tank now have been in the tank for nearly four weeks without much deterioration thus far.
I simply soaked mine in a five gallon bucket for a couple days and then tossed em in the tank.
Those I picked up yesterday, I placed in three gallon freezer bag.
I have found that cory's,pleco's,loaches,and numerous fishes use these leaves to hide near or search for infusoria such as Byron mentioned and fry can hide among leaves as well.
I placed eight large leaves in 55 gallon and with twice weekly 50 percent water changes due to waste from pleco's,,neither the wood,or leaves make much of an impact with respect to hardness levels.