I had one Malawi eye biter in my 150G. He was about 4" and mixed with Peacocks and Haps. After 3 months, I was still deciding if I wanted to get rid of him or get another. He had an attitude and kept me on edge because I had a relatively non-aggressive mix of African Cichlids. He wasn't chasing the other fish, but would sneak up on anyone who entered his territory and make a quick charge at them.
Well I made my decision when I added my new Hap (Protomelas Taeniolatus). Even though the Hap was about an inch longer and much "heavier", the Eye Biter went over to challenge him. They had their bodies parallel to each other and doing a lot of shaking. I watched in horror as he took out the right eye of my new fish. Off to the lfs he went within microseconds.
So don't believe those that say they are harmless. If you don't believe me, you can buy mine as he is still unsold after 2 months at the lfs.