Scientific Explanation
All most all fishes have the shoaling behavior, The word shoal, on the other hand, is the term for any simple social grouping of fish. Shoaling is mainly for
increased success in finding food access to potential mates, increased protection from predators
With respect to food acquisition, it has been shown that fish in shoals find food faster and spend more time actually feeding. This benefit probably stems from the simple fact that a large group of fish has a greater chance of finding food than does one single individual. With respect to mate acquisition, shoaling behavior often results in groups of fish of the same species, thus making it easier for males and females to find each other. For both food acquisition and mate acquisition, larger numbers of fish should lead to increased benefits, therefore it is not surprising that fish typically choose large shoals over small shoals.
More details are available in the scientiifc literature http://www.sju.edu/int/academics/cas/biology/resources/biodiversity/fishcam/shoaling.html
