I have an african brown knife...great lil' fish, but very shy. He got along well with my bichir...so did my leopard ctenpoma, but they are a pain to keep, the require very, very small fish to eat and they are stubborn about pellets. I'd get them again though....I have a hujeta gar in my 55 right now...cool fish, he'll eat everything in sight if its under 2" though. You could get a pair of earth eaters, I'm a huge earth eater fan. African butterfly fish are pretty cool and stay near the surface...(so does the hujeta). You really are wide open, just look for fish under 9". thats about the max in a 55. You could get a zig zag eel...they might be a lil' big though...it may compete with the bichir...who knows. Is there any certain fish you REALLY like? big or small?