You should never house more than one black ghost knife in the same tank, or house any 2 fish that use an electric current. when you keep themn together they try to kill eachother, and they get confused and cant navigate and find food properly.
black ghost knife are pretty sensitive to water conditions.
After mine died its fins became very frayed.
And they werent like that before it died. it had not been left dead in the tank very long. this fin is very important to the fish to swim around. either the previous BGK's did it, or it wasnt like that before it died.
Was it eating properly? what were you feeding it. and what are the parameters of the tank? (ph too)