Aggressive fish are species known for their territorial behavior and tendency to attack other fish, especially when they feel threatened or are protecting their nests. Common examples include the piranha, betta fish, and cichlids. These fish often display vibrant colors and unique patterns, which can be a warning to potential intruders. In aquariums, aggressive fish require careful management to prevent conflicts. It's essential to provide ample space and hiding spots to reduce stress and aggression. Understanding the specific needs and behaviors of aggressive fish can help ensure a balanced and healthy aquatic environment.