lol
There is a HUGE difference between all of the common knifefish. This is one of those things you really should get the full info on what the thread starter is talking about because all three of them differ greatly and I have seen african knife fish (Xenomystus nigri) and ghost knifefish (Apteronotus albifrons) labeled as "black knifefish".
There are usually 3 types sold:
-african knifefish (Xenomystus nigri)
-clown knifefish (Notopterus chitala)
-black ghost knifefish (Apteronotus albifrons)
Clown Knifefish get around 3-4 feet, are highly predatory, and are definitely NOT a community fish at all.
Ghost Knife fish get around 2 feet I believe, and are actually quite docile. But they will eat anything that will fit into their mouth. And they don't defend themselves very well so they should be kept with large, docile fish.
African Knifefish only get around 8-12 inches usually. They are also docile, but will eat small fish that fit in their mouth. THESE are the knifefish most people should get because they don't get monstrous. You can keep them in a 55 gallon tank easy.