Just wondering your tank size. Did you know that ghost knife fish can get easily as big as a common plec. I think what I heard was 20 inches. Just so you know you may have to rehouse him depending on your tank size.
About 3 years ago when i was living somewhere else, my tank had a BG and about 3 angels that where the size of my palm. Just as long as the BG has a cave or somewhere to hide, they do well. Angels mostly stay at the top, well mine did anyways!