Welcome to ca/sa cichlids! That's what they do. All of my fish do it at some point. As far as aggression, it's been hit and miss for me with nics! (wow that rhymed far too well) I've had some before that were ok. I now have one, 6"ish, I've had for a couple weeks, and he is the most fin-nippingest sob I've ever seen! He's like a big colorful tiger barb! 
