Clown knives are great tankmates with arowanas, just make sure to provide a decent amount of cover for each, driftwood and/or plants for the knife and floating plants or dim corners for the aro, considering they can both get kinda jumpy. In my experience, Clowns are great with anything that cant fit in their mouths, though id advise against keeping with redtail cats from the horror stories ive heard. I have a royal clown knife that is the baby of my tank, about 7", in with a 10" silver aro, a 17" spotted gar, a 14" tiger shovelnose, a 12" polypterus senagalus and an 8" silver datnoid, and the knife is thriving. Keep an eye on the actual eating capabilities of the knife when choosing tankmates, however. If you ever see a knife "yawn" and unhinge its jaw, it is quite amazing what it could fit it there.