Control Your Fighting Fish
What characteristics do fish possess that cause them to fight? What upsets them? What can you do to keep your fish from fighting?
One might consider the "Fish are like humans" approach to understanding their aquatic behavior. They have attitudes and social tendencies, just like you. However, as with humans and empathy, you can avoid fish aggression by introducing several ideas into the way that you set up their environment.
First, obviously size is important. Try to match species with similar personality traits and see that fish are relative in size and growth rate. A good idea would be to integrate the fish at an early age; when they are all rather small and will have less aggressive tendencies. The idea behind this is that fish are territorial, as with most species. Try establishing separate "sides" or two different environments relative to each fish's natural habitat.
Another aspect to consider is a group of fish vs the isolated ones, when combining them in the tank. It is common for some fish to possess very aggressive tendencies in groups but appear very docile when isolated from others of its kind. A group may attack; whereas separated they might grow bored and lose aggressive habits over time.
Finally, what you feed your fish can also have an impact on its behavior. Peaceful fish usually depend on a stable diet, while aggressive ones should be provided with a varying diet to promote better health and more activity. Keep this in mind when combining them inside the aquarium environment as well. Naturally, to introduce different feeding habits or the wrong habits to the unintended species may have adverse results to the fish and its tank mates.