I keep my 8"ghost knife in a 125 with mixed aggressive fish. As long as he has a good spot to hide. And the other fish aren't big enough to eat him you'll be fine. I have 2 6"oscars, 2 8" iradesant sharks, 2 12" alligator gars(spotted gars) a 5" Dempsey, and a large pleco. Never had any issues.