My how this place has gone downhill. And no one thinks to ask what the size of the tank is? No question on the conditions of the tank? How old is it is? My heart is there for big fish dude. So take heart to what I say cuz I'm only posting once. These fish need big tanks. To put it in perspective my 150 gallon may be long enough at 6' but 18" is too narrow of a width. Research their usual hardness they need. And look at what you are keeping it in. You shouldn't be keeping BGK with an oscar long term as is. Oscar is going to get big one day and he may not get along so well with the ghost knife. And if you are keeping in in a small tank, shame, dude, shame.
I don't care if you plan to get "bigger one later" because plans fall apart. Also these are super sensitive fish. The are scaleless fish, and stress easy, and can get sick easily when this young.