This is a discussion on mean one within the Cichlids forums, part of the Freshwater and Tropical Fish category; -->
I know that cichlids are territorial in tanks but I have one that is chasing everybody around the entire tank like he owns it. ...
What kind of cichlids do you have and what size tank? I have a bunch of cichlids and as I was adding them to the tank the new ones would get chased around for a day or two until the new guy would find a spot of his own. So after a while my tank has gotten pretty peaceful with the occasional chase off but from what I've read about cichlids they can be fine together for a long time and then decide all of a sudden to be territorial and mean. You might have to take back the one that is picking on all the others. We had a green terror that was fine for a week or two and then became really mean and was torturing some of the others so we took him back and traded him for a firemouth that is totally peaceful.
I am not entirely sure of all of their names but they are mbuna african cichlids and the mean one is blue with horizontal black stripes. they have all been fine until today and it is just one of them that is being mean out of 13 and they are in a 55 gal.
You can try taking the problem fish out, rearranging all of the decor in the tank, and then adding him back. This may reduce aggression somewhat, but you might end up needing to trade him for another fish that isn't so problematic.