Hozelock manufacture a range of high quality pond air pumps that come supplied with air line and air stones. It is a good idea to use additional aeration not only in the pond but also in the filter unit (gravity filters only). Additional aeration in the filter will allow more aerobic bacteria to colonize it making it that much more efficient. Extra air in the pond will help on hot humid nights when the risk of low oxygen levels could lead to fish deaths.
Guide to Hozelock Air Pump performance