If you like the present gravel, it will be fine as a substrate. Sword plants are probably the best in this situation, and they will root in gravel unless it is very large.
Swords are also fine in moderate light, something essential for the Black Ghost Knifefish
. I would also get some floating plants if you don't have any, species like Water Sprite
is ideal, or Brazilian Pennywort
left floating works. Again more shade for the BGK; this is a nocturnal predator by nature and will be less stressed (and thus healthier and out and about more) with minimal light and plenty of hiding places in wood, PVC pipe, etc.
I should caution you on these fish, in case you are not aware of it. The BGK and the Tiger Oscars will require much larger quarters. The BGK is in our profile [click the shaded name]. The Tiger Oscar
is a man-made variant from one of the natural species and attaining at least 12 inches and often more, needs twice the tank space they are now in. If both are males, one will be dead when they mature a bit. If a pair, they will obviously spawn and be nasty to the BGK who is not going to stand much of a chance. Just so you know.
And, welcome to Tropical Fish Keeping forum.