Sorry for your loss of such a excellent fish. Were you feeding shrimp with the tail on? I have heard similar stories of Aro's becoming septic do to perforation secondary to feeding shrimp w tail or heads on.
no, i would feed with shell on, shell is good for them.
but i always rip the sharp tail bits off and remove the head.
i think perhaps when the temp dropped, cos i do feed the shell, it was harder to digest.... dnt really know, its hard to say.