Black Ghost Knives are a nocturnal species. They can be shy at first but show themselves more when settled. They need several hiding places (eg driftwood, cave like structures, dense planting) - the more hiding spots, the more you'll see them as they will feel secure enough to come out.
They can be sensitive to medications as they are a scaleless fish.
They are a naturally predatory fish, but if fed well and kept with larger, peaceful species they may leave fish alone. Avoid keeping them with fish that will fit in their mouth.