Keep arguing, SBDTHUR, but you are wrong.
The knifefish in that picture is only 6 inches long, not 12 inches. You may not be able to see just how large its mouth is by watching swim around in a pet store tank, but when they want to eat something they can open their mouths very wide. They are known to pick off community fish and disagree all you want based on your "small mouth" theory, but they are predatory fish.
And the idea that fish grow to the size of their aquarium is BS. Fish become stunted and die prematurely, but a BGK should have no trouble reaching 12 inches or more if it is well taken care and in the proper size tank.