The meanest fish i kept were my Tilapia buttikofferii's, they were nasty with a capital N. The meanest i have now is my jardini arowana, infact it is probably meaner than the butti's but just hasnt done the same ammount of damage to another fish, yet. Another mean fish i had was a black shark, the bigger nastier cousin of the redtail black sharks, it kept itself to itself most of the time but god help anyfish that swam in front of its cave! My leporanis are quite mean too, theyre only juvies but have already killed two FW baracuda's and a pike cichlid and shredded the fins of a small shovelnose catfish though they seemed to have calmed down now.