I second that motion, go back to Tolaks suggestion as it sounds the best, but first remove the adult BNs to another tank (or a net breeder if you've got one) that way the babies get the food and presto!
Or add two pieces of cucumber, one in the glass/mug and the other free. Surely the adults will attack the easier piece and hopefully you would think that the younger plecs will dive into the glass out the way of their parents.