Why not an arowana they stay mostly at the top. I would get one bigger then your cats though, also love the clown knife as well. I have a 600 that will soon be housing a 20" arowana and a 18" RTC with a TSN growing in a smaller tank he's about a foot now. I had a clown knife I was trying to grow out but he died. Also have a 14" motoro ray going in with a 14" alligator gar. Still looking for a large clown knife. The arowana im getting is very agresive and think he will be fine, i had a smaller one that was really shy and be scared when I came to the tank and he got eaten so make sure he is not shy when you approach the tank