What is the most aggressive aquarium fish?

The most aggressive aquarium fish is generally considered to be the African cichlid, particularly species like the Peacock cichlid (Aulonocara), Haplochromis, and Nimbochromis. These territorial fish are native to Lake Malawi and Lake Tanganyika and are known for their fierce behavior toward each other and other less aggressive tank mates. While African cichlids can form peaceful communities when kept in spacious setups with proper species pairings, they require careful consideration and knowledge to establish and maintain a harmonious aquarium. Other fish known for their aggressiveness include tiger barbs, red-bellied piranhas, convict cichlids, oscars, and some larger gourami species like the Siamese giant gourami.