100% sure. If a fish gets attacked by an ATF, it will have long scars / missing chunks of tissue. The dead pacu had 'round' bite marks on the side of its body.
Figured I'd end this thread with an update. Earlier this year, I upgraded my big aquarium and while doing that I had to move the sheep pacus in with my big guys in a 180 for a week. And during that time my big VATF and the Armatus decided to chomp on the remaining two guys, and this happened:
Pacu #1:
Other Side:
Pacu #2 was missing half of its tail:
Pacu #1 didn't survive and died soon after. The other pacu recovered completely . But I noticed a funny thing. It became extremely skittish and would just jump around banging its head to the aquarium's walls all the time. Thats when I decided that I should sell the last survivor. Now he is in a nice planted 120 gallon with tiger striped silver dollars .
My experience in keeping these guys as a tl;dr:
I had 3 and 2 of them decided to kill 1 guy, the other two fought with each other often. If I get these guys again in the future, Ill get like 7 of them and see if works out better.