Yes great advice, why dedicate 1000-1500$ to a giant pacu??! A 500gallon tank has so much potential wasting it on a very common overgrown giant is such a waste, im not saying fish do not deserve proper space but this was thrown out the door before it was brought to a 10gal then a 55.
There are some mom and pop fish stores in my area that would take him, you prlly wont get any money or credit for him but atleast you know itll go to a decent home! There are thousands of interesting colorful fish that could beautifuly inhabit your 55gal with no problems.
Imagine a mountain bike wheel and tire, that is how big your pacu will get, now think if you want that commitment for 5-10+ years!!
If you decide you can handle it then more power to you! You can also try putting him free on CL and requesting pics of potentiol buyers tanks to make sure its not going backwards!
Goodluck
6 rbp