Has your tank got any hiding places in it. Most fish need a place that they can call home - a cave, under a large plant, somewhere they can hide from bullies. If you have a planted tank, this helps to eliminate eye contact between different types of fish. The plants don't have to be real, although I personally prefer them as they help to oxygenate the water, and if there are no plants, then caves, rocks or ornaments where fish can go to get a bit of "peace and quiet".