Unfortunately, this isn't always possible, and if you have a fish that is truly too aggressive for your tank, your only choices may be to try changing the decor in the tank around, adding additional hiding places, returning it to your supplier, or exchanging it with another hobbyist whose fish may be more compatible. Before you make that choice, though, and to help you to decide about compatibility issues, you will first want to examine the possible reasons behind the aggressive behavior.

If you have chosen a good mix of species to live together harmoniously, provided them the right amount of space, and given them sufficient food, one or two rogues may still pick fights or chase after tank mates. If this is the case, you may try isolating those fish for a brief period in separate tanks. When such a character is returned to the communal tank, he'll find it to be a less familiar environment, and he may act less territorial. If that doesn't work, then a permanent separation may be your only option.