It sounds like aggression over the food in the net to me, I had a neon blue gourami who I had to take back to my LFS because he guarded the fry net to the point where no fish could go near the front of the tank
I don't really know how to help you here, sorry I don't know whether moving the breeder net will work, but it could be worth a shot, even if it's only a temporary fix. Someone else should come by here and be able to help.
I haven't ever heard of older fry guarding newborn fry.... But maybe someone else has.