Damsels are very common in the warm waters of the Atlantic Ocean. The average size of most Damselfish is less than 3 inches. They come in a varity of colors and are tough. They are often used to break in or cycle new aquariums. The majority of these fish are highly territorial, and may become aggressive toward other tankmates, although they do well in groups of the same species. They will stand their ground and protect their spot in the tank at all costs. In most circumstances it's best to only have one to an aquarium.