Peaceful corals are quite passive in their existence and vulnerable to being killed by more aggressive corals. The corals below are all peaceful.

Semi-Aggressive corals can (and will) fight for superiority with other corals in your aquarium. Be sure to give them the space they need so they don't feel threatened. The corals below are all semi-aggressive.

Aggressive corals can dominate an area of the aquarium (or the whole aquarium) and kill off other adjacent corals as they grow. They are adept at defending their territory and will do so, so be sure to give them plenty of space to grow. The corals below are aggressive.