I think the black ghost knife is a night time predator, I had a knife fish once and I would find mystery deaths every now and then. I never knew what happened to my fish dying, but I read on here that they are pretty aggressive. I found half a rainbow shark once, pretty sure it was the knife fish.
Sounds like you had an abba abba knife. THOSE things are aggressive. Africans are quite timid, and most peoples accounts of bgk is they are also shy. I have not had my bgk long enough to comment on it's adult temperment, but as a juvi mine is shy.
He was very very shy during the day, but if I stayed up at night with a full moon he was all over the place. He spent most of his day under that rock. The only other agressive fish I had was the rainbow shark, at least that I know now.
bk's are most deff preditors at night time...especially as adults.....I dont think a 75g can house one as an adult either...nor will it be room enough for clownies if you have the other fish....I think the angels, rbs, serpae's, and some khuli's would make a fun tank..at .have some silk/real plants with big leaves that are taller, would help also..not sure on the ropefish..never kept them..although tigers are nippy, if you get a big enough school (6-10) they may do ok.....good luck!