my 13" fire eel, Pompeii, is definatly tank boss when it comes to the eels and there was problems when I first added the smaller fire eel but it has calmed down and I feed him 1st to distract him from the smaller fire eel during feeding time. As far as him messing with my new eel I've only seen aggression once and that was to tell my new eel the tube he was in belonged to him. I do keep an eye for aggression though