Is the GKF a Black ghost knife fish (jet black with a white tail)?If so it will grow to 15" is peaceful and needs a daytime hiding place as it is nocturnal,should not be housed with aggresive or overly active tankmates or it will become stressed.Needs perfect water conditions at all times and is sensative to nitrates.
The second fish could be either a Black upside down catfish (synodontis species),i tried getting some info but had no luck on google,ill have a look through my books later.Or it could be a black lancer (Bagroides macracanthus) i have posted a picture for you in members aquarium pictures,if this is your fish then it is a nocturnal species grows to 16" needs a heavily planted tank with many hiding places and a gentle current,not a fussy eater and is peaceful to even tiny fish.