I rescued my Pacu from a friend of mine who bought the fish for their 10G tank of raw sewage(it was setup about 3 years without a single waterchange...just got ripped down time to time), she had the Pacu a couple of months it grew from less than an inch to 3" and had fugus, some sort of bacterial infection and hell knows what other sorts of disease. She was just going to flush the fish and buy another instead of treating it...I took him home instead. I put the fish in a 20G long...with good water quality, and treated its diseases. Took a good week or two of constant medication and after he got over that he developed ick...so i treated that. I had that fish a bit over a year....it nearly died on me a hundred times the first month I had it....but now its an absolutely beautiful 12" or so fish. Ive never actually measured him tho.
Despite what most people report about Pacu being skitish or flighty...mine is totally chill and has NEVER flipped out on me. Hes very pet-like and owner responsive, hes never messed with my decor and my 150G tank has alot of driftwood and rocks and things. I expect that the 150G will do him for another year to year and a half depending on how his growth rate goes between now and then. Ill let him get to about 17-18" before I upgrade. My tank is a custom size and has a 20" width so he should be good for awhile longer. I saw a tank at my LFS it was a 225G(72*30*24), they only wanted 700bucks for the tank, cabinet and canopy....and that would be a decent home for a Pacu for almost its entire life if not entire life. Most redbellies in the home tank rarely get over 24", and 30" seems to be an extreme rarity.
I think your Pacu is a lovely example of the species. Depending on how your fish was treated in the past and its genetic make up will affect its growth rate. But no matter how **** like either of those are....the fish will definatly need a larger tank in the near future. Definatly get something 30" wide when you do upgrade, even if you can only go 6ft in length...the ability to turn is more important IMO.