The scratch pool, if defined, and you can only have one, btw, is used by all pools without exception. However, any pool would consider an ACTIVE media in its own pool first which has the same retention settings (unless you allow mixed retentions per media) before dipping into the scratch pool.
Depending on the media state you may be able to move them to the scratch pool.
Go to Media and show the status column, all media that is AVAILABLE can be moved to Scratch (right-click Change). Alternatively - sort by Assigned date and choose those where Assigned date is empty (this means these tapes are unassigned)
All other media that is ASSIGNED (i.e. Active, etc) cannot be moved until all images on this tape is expired and it becomes AVAILABLE