that is suck and how big of pacu u have?
i had to return red belly pacu back to the LFS, that fish is very energetic and hogged all the food LOL
my aro did not like the pacu but pacu saw my aro no thread to it, good thing i returned the fish after a week, i cant let my bichirs food taken away from their mouth.
Pacu are a social fish, keeping one solo will cause it to act oddly, this includes aggression and perhaps eating your arowana.
They really need to be housed in pairs, trios or more.
A 90 gallon tank is way too small for a 13" pacu. He will break the glass of the tank killing himself and any additional tank mates soon. I hope you can get him into a larger tank with a room mate or 3 soon. He'll thank you.