On top of the reef are often large schools of fish from different species mixed together.
It is actually quite common for fish to stay together in groups, it is estimated that half of the fish species do it for parts or all their lives.
For the nerds: in English a group of fish that stay together for social reasons are said to be shoaling while a group in which the fish are swimming in the same direction in a coordinated manner are said to be schooling. – Perhaps you do not have that distinction in your language – but try to look at the fish you see. If you see e.g. a group of Lined bristletooth or fusilier (see Schools of silver), you will often see them busy feeding from the reef rather uncoordinated, yet moving in the same direction – they are shoaling. On the other hand – during the day you are most likely to see the Yellowfin goatfish (see Over the sand in the book), Doublebar bream or Ehrenberg’s snapper (see Along the wall in the book) swimming very coordinated and almost synchronic in their behavior.
Why stay in a group?
As mentioned in the book, there is an obvious advantage to be part of a group as a defense against predators. The individual fish is hard to select and focus on when among similar individuals. That is why the schools of fish mostly consist of same species or species that are similar in color and size.
Some of the predators are e.g. trevally or barracuda (see Silvery fish in the blue in the Book). As you can also read here (more material on Silvery fish in the blue)
Source: https://en.wikipedia.org/wiki/Shoaling_and_schooling