A simple group of fish is called a "shoal." It can contain a number of species. Shoaling makes it easier for fish to find food because more individuals are on the lookout. Large numbers also help fish find mates. In addition, being in a group offers some protection against predators, similar to hiding in a crowd.
A group of the same species of fish that swim together and move in a synchronized manner is referred to as a "school." About 80 percent of the known species of fish participate in schools. Swimming with their relatives helps fish move through the water more easily, because it helps reduce friction between their bodies and the water. Predators are sometimes confused and frightened by the large body. As they move in precise formation, schooling fish take on the appearance of a single huge creature.