Here is something I have never come across: one little fry who will be culled. I noticed it's irregular behaviour quickly. Rather than football spirals, common in fish with damaged organs (dropsy), this fella has no physical problems, nothing visible. However, he darts in small circles, which is commonly seen in dogs with tumors or neurological problems who walk in small circles. This is exactly what this fishy is doing
and I have never seen it in fish. However, connect that with the red in some of the dead fry's heads (not ON the skin but rather under, with the brain) could it be possible they all had problems like that?