I wonder if one of the reasons they are deemed aggressive is because the Carapo knife family is much different then the other less-aggressive knives.
The difference between say Carapo knives, and Ghost knives is this;
Ghost knives can be farmed, collected, exported, imported, and wholesaled in large groups and they will TOLERATE each other.
Carapo knives can be farmed, collected, and exported and by that time many will perish and die because they DO NOT TOLERATE each other. I believe that many importers try to keep the Carapo knives in groups just as the ghost knives will tolerate each other, but come to find out the carapos will fight until the death. They are definately aggressive towards their own species, and should be seperated. Perhaps since this is a rare fish, more often seen at an import/wholesale facility then in the actual hobbyist tank, that the Importers themselves have given them the label of it being overly aggressive, because they kill each other if they are not isolated.
I believe there is just too little of information to say these guys are potential aggression problem in a community tank setting. I also believe that most people who own these specific knifefish have paid such top dollar and are so intrigued by their wonderful specimen, they are not willing to take the risk of housing it with tankmates, isolated it as a single speciemen display tank.. Which would then lead others to believe they are overly aggressive, and should only be housed alone.
I also wouldn't be surprised if someone had one once, tried to put it in with a big clown knife, and it killed the clown knife.. and then they ran around telling everyone "I have this killer knifefish it kills everything"
I would be more worried about the knife being picked on than anything. It dosn't have the capacity or mouth structure to damage another fish.. It may have the ability to harass other fish, especially at night, but any experienced fishkeeper (paying that kind of money) should be able to monitor and alleviate any situation that could arise.
