A school is a group of fish of the same size and species moving in the same direction. Schooling may increase the chances of finding food, avoiding predators and finding a mate. It may also make swimming easier. Sight is important in keeping the school together. The lateral line may also help.
Aggressive behavior is an interaction between two fish of the same species or different species.
It is usually associated with reproduction and defense of feeding territory. It includes posturing, direct attacks and ritualistic displays such as fin flaring and changes of color. During the breeding season some fish protect spawning territories. Fish don't sleep like people, but some fish spend a good portion of their day or night resting. This may reduce competition, conserve energy and keep the fish safe from predators.