You need to have a scratch pool. Create one the way you created the other pools but name it scratch. Make sure you tick the selection box "Scratch Pool". Remember to activate barcode rules.
This will direct all expired/deassigned media to the pool, the process is automatic. NB will the pick the media from scratch pool and assign them to the relevant pools as needed. In simple terms, you don't have to interfere.
The retention period will cause the image to expire even thought the data still exists on the tape until it is overwritten.