For pacu to thrive, that is to live almost to its full potential, the fish is best suited to grow in ponds.........Now I have I seen this fish grow in many different sized tanks...........I had a local shop keeper that kept a Pacu in a 180g for almost 15 years..........Healthy, the fish grew two 2 feet long and was thick........I have also seen this fish grown in 55g tanks.......Within a year, the fish grows to a foot long, most of the time, give or take a couple inches..........The fish is then returned to the aquarium it was bought from.......I would have to say minumum size tank would be a 300g.......8-9 feet long, and whatever so wide.........My uncle had a 9 foot 300g......I raised two red bellied pacu from 2 inches to over 2 feet..........After 3 years, he decided to donate the giant fish, along with red tail catfish he had, to the Steinhart Aquarium..............
That is close to the footprint of the tank I kept a pair in at one time and it would have been nice if it had been bigger.
I probably would not consider them again unless I was thinking of a 1000g plus set up. They not only get big but are incredibly powerful and hard to handle if you have to move them.