Well every coral is "potentially" aggressive. What kind of corals do you want to keep? You need to keep in mind that some species of corals have feeder tentacles that can get quite long and constantly reach out to sweep food from the surrounding area. Some corals just don't like to be placed too close to others as they will form "sweeper" tentacles that can reach out and touch other corals and burn them. A good example of both of these is a hammer coral. I have a hammer coral in my tank, I just keep a coral free radius of about 6 inches around it so it can't reach out and burn other corals. A lot of LPS corals operate this way.
Other corals become aggressive when they grow too close to one another. When they get close enough, the polyps can reach out and touch other corals and cause problems.
Basically you can keep anything you like, keeping your lighting in mind of course. You just have to think about the physiology of the coral and what kind of action it will take when it gets close to other corals.
Hope that helps
Christy
