You never said you sold it http://www.monsterfishkeepers.com/forums/threads/black-ghost-knife.666593/ , but only that you may sell it. Kind of ironic, I just got one yesterday. It hogged a whole bloodworm cube at 2".I sold my BGK because well I never saw it lol literally never saw it eating or saw it. Good luck
A common 55 gallon tank dimensions are 48 x 20 x 13. A bgk gets to 18-20" on average, so you wouldn't be able to keep it in that tank happily for the rest of its life.