I have a 120 gallon fishtank and my pacu ate my pleco?

i have a 8 inch pacu and it ate my 6 inch commen pleco why is my pacu a.k.a professer p just really aggressive (i know pacus are peaceful..) or does he just need 1 more pacu in the tank (i know the pacu will need a much much much bigger tank then a 125 gallon) plz help