CaseyL, aeration and Bacteria will make the leaves and needles decay faster, but will not remove them. It will only slow the speed of the buildup. Most aerobic bacteria's are already present in a pond, adding aeration provides the dissolved oxygen needed for the bacterial colonies to thrive.
Since this is a concrete pond, it may be easiest to use a trash pump with a weighted intake strainer and pump the muck out of the bottom. Adding the aeration will certainly lessen the pace of new buildup, and improve overall water quality for you.
