|The ideal situation for incorporating two or more aggressive fish of the same species is to add them simultaneously. This way, the fish can establish their own territories with minimal conflict. When you add an aggressive fish to an aquarium with an already-established, territorial fish of the same species, attacks are practically guaranteed. Because the already-established fish considers the entire aquarium to be its territory, it will fight to protect its territory when new fish are introduced.
You may wish to try the "mirror trick" when acclimating a new aggressive fish to an established aquarium. Simply place a mirror outside of the aquarium so the established aggressive fish can see itself. This fish will then be preoccupied with its own reflection, giving the new fish time to establish its territory without excessive hassle and hostility.
Since aggressive fish need their own territories to "guard," be sure that your aquarium contains many hiding places that can serve as individual territories. If space permits, you can add large amounts of live rock to your aquarium, then carefully arrange it to create lots of caves and hiding spaces. If you already have live rock in your aquarium, rearrange it. This will effectively eliminate established territories and give all fish an equal opportunity to establish new territories.
Aggressive fish will also fight for dominance over food. Because of this, it's a good idea to provide more than one food source in your aquarium. Tangs, for example, will benefit from a vegi clip with dried seaweed at both ends of the aquarium.