I'm a new owner to black sharks and am no expert, but have been doing a lot of research. Black sharks are pretty agressive, particulary with their own kind and fish that look anyway near like them. I have tried a lot of tank mates for my B S, and only have success with small tetra, barbs and rasporas. As i have read and experianced, they don't mind small tank mates, but do not like bigger fish.
Mine has bullied/attached: cory cat - half his size, large sword tails and platies, 2 A.sajicas, algae eaters, tiger barbs, and a pleco.
Granted, mine is only about 5" and is in a 25G tank with 6 small gold tetras (going into a 55G next month). Had it for about 2 months now, but have been learning and observing as much as i can on these cool fish.
I don't think you should put your B S in your 55G. How big is it?
Also hear it takes a long time for them to reach adult size. I have noticed mine growing slower than i thought. Eats well, nice and plump but hasn't put on much length since i've had it. How long have you had yours and what size is its current tank?