Hey what's up man. I had a 5 inch or so black ghost knive with my jardini in a 55. A few years ago. Untill the jardini was around 13 inches when i moved the jardini into a bigger tank. And i gave the knife to a freind. But they got along and the jardini diddn't bother him. the knife used to swim around with him for a little bit each day was weird but cute even. Cant speak for what would happen at bigger sizes. But silvers live peicfully with parrots and other fish that are much smaller then a full grown black ghost knive so i dont seen a probaully good luck. Ghost knifes are awsome fish. Help that helped.
Sincerely,
Jeffrey allen Braley