Since you've pp'd the fish, why not do the same for the sashing? At least to get the size of pieces and layout of pieces you'll need to connect the blocks. You can lay out the paper parts to make sure seams intersect where they look best before cutting into fabric, which could be done just by measurements at that point if you'd prefer, or by finishing the pp'ing. With the staggered blocks you may not want full length vertical or horizontal seams in the sashing anyway.
Then bind in at least one of the other colors in the pp'd fish.