I've had all those fish, the barbs will fin nip something fierce, all barbs do ! The red tailed shark could get a little overly agressive towards the other fish, I recently had to give mine away, he got bigger and meaner and he is with two 6" Severums and a 7" Jurapari and an Electric Blue Jack Dempsey*twirlysmi ! and the black ghost can get huge. When it does get bigger it will eat other smaller fish. I used to put feeder rosies (goldfish sorta) in my tank to feed another fish and the black ghost would always steal one for himself. I've done some crazy things but I don't think I would have a black ghost in a 20 gal. aquarium. Don't know how fast they grow, but you would have to part with it at some point. *twirlysmi