1. When adding particularly aggressive fish to your aquarium, it is a good idea to move around the rocks and such in the aquarium before placing the new fish into the aquarium. Moving the rocks in the aquarium will allow the fish to go back and stake out their own territory.
2. It is important to pay attention to the limitations of the fish you place in the aquarium. Do not place fish into aquariums smaller than there recommended range, this can lead to increased aggression and decreased health.
3. Some people go by the ìinch of fish, gallon of water rule when deciding on how large of an aquarium to buy. This is a completely false and bad method to go by, you should always check your individual fish's tank requirements.