i say why not. what's dead is flesh. as long as you are hungry. however, to eat it, i suggest killing it at a prime age. old fish dont taste as good. Meat's rough. and death from diesese is obviously a no no...
I ate a pacu by accident once. they were selling them at the local wet market. I saw what looked at a glance like my piranha in the fridge... domestic helper (maid) had bought it. I think it's become a food item, probably spill over from pet trade in china. Meaty and hardy, grows fast. Must admit it felt a bit weird but tasted fine.
They sell something that looks like a snake head/channa cross here as a regular food item. Bought a couple and threw them in my pond while back, hid, never saw them, died over winter.
... no i didnt eat those. =)