Shweet tank man. How big are your ghosts? Just make sure there is enough hiding place for your BGK's coz they do tend to be territorial and the arowana might just regard them as a snack. Otherwise, awesome tank man!!
Looks like you have enough roots in your tank. Black Ghosts love floating vertically in plants. Mine loves my java ferns, vallisneria and anubias for some reason. Dunno why they like it so much. Also you can get any cave-like decorations. In many pet stores you will also see the fish chilling in plants or in plastic pipes. Personally, I don't like plastic in the tank so much, so get some nice plants and rocks. I think it would really make your tank look stunning and give it some nice colour.