Black Ghost Knife Fish are quite cool beasties - though be warned: they're meat eaters and they can grow to about 18". It's also not recommended to have more than one per tank: they're part of the electric eel family and use electric currents to navigate, so if there's more than one, they're likely to permanently stress each other out.
They don't grow that quickly (the baby we bought has doubled from 3" to 6" in about 3 months, while the adult in our other tank has gone from 5" to 8" in about 7 months). In theory, you can get them to feed from your hand - I suspect this depends on the number of other fish in the tank, as ours have remained quite shy - except for when there's bloodworm to be had
. You also need places for them to hide.
On the other hand, for all that they're carnivourous, we've not had any problems with them eating other fish - even when left unfed for 2 days (not something I'd recommend doing on a regular basis, admittedly!).
Both Fish Inc and Aqua Hobby have/had BGKs in: it may be worth having a quick chat with them...