Dont need to set up a new prune job, just enable normal database pruning.
At the Media Pool Configuration use the Prune Retention Time (days) option. Specify how long you want to keep detailed records about the backup jobs on the tapes in the selected media pool. After the number of days you specify has elapsed, the records on this media pool will be pruned from the database. If you select a prune retention time of zero, this information will never be pruned from the database. Note: This option takes precendence over the Prune Database Records Older Than _ Day(s) option.