you mean it's not normal to keep a fish in a tank that grows 300lbs???
Honestly, in my professional opinion if I have to have one lol, I would only keep it with other large catfish, turtles or Crocodilians. There's virtually no chance of other FW fish retaining a healthy happy lifestyle with these brutes. IF the Paraiba, Croc or Turtles were all the correct size meaning the crocs and turtles would not only have to be big enough and similar enough growth rate they could never be taken into the catfish's mouth, it would equally have to be vice versa for the Paraiba as well.
My advice, keep them alone. Do you know how big a tank for a Paraiba and a Black Caiman full grown would have to be? I'm going with a minimum of 40ft any direction