I've seen a huge clown knife in a 1000 gallon tank and it looked fairly happy, though it was sharing the tank with a close to full grown silver aro and close to full grown red snakehead. there were also some large pacus and huge cats as it was an lfs display tank. having seen these fish though at these sizes I could easily see how they definitely need huge tanks to be happy.
and I agree that such fish as pacus, clown knives, silver aros, TSN's, RTC's, irridescent sharks etc should not be as common as they are. what's worse is the info that's provided for them. I can't tell you how many lfs I've been to that have the little tags on the tank saying pacus grow to 8".