The importance of living in a shoal?
In the wild, bigger fish eats little fish. If you are a little fish then you spend a lot of your time worrying about getting eaten. One way to combat this is to live in a big group with other little fish, that way there is less chance of getting eaten.
In the aquarium small fish are usually safe from predators - however it is difficult to tell that to the fish. This is why in the aquarium small shoaling fish should be kept in groups of the same type. For most small species a group of 4-6 is sufficient.
