Not about fish
In Biology, fish shoal (/shohl/), stay together, for social reasons, deriving many benefits from it: defense against predators, enhanced foraging success, and even higher success in finding a mate. Fish even choose shoalmates of their own species, similar to them in age, size and appearance. When a group of fish swims in the same direction and speed in a coordinated manner, we say they are schooling (/ˈskuːlɪŋ/).
Fish can be obligate or facultative shoalers and can shift from a disciplined school to an amorphous shoal within seconds. Fish schools tend to have minds of their own and, although the intricacies of schooling are not yet fully understood, there are rules to this arrangement: move in the same direction as your neighbor; remain close to your neighbors; avoid collisions with your neighbors.
A school isn’t defined by a magic number, in the wild, schools of fish can often number in the hundreds or even thousands. In captivity, a comfortable school of fish contains between four to six. The calming effect of being in a large group is a powerful social motivation for remaining in an aggregation. The more, the merrier, right? Nonetheless, this behavior is rarely displayed in aquaria. Even with the best facilities aquaria can offer, fish in a bowl become fragile and sluggish compared to their quivering energy in wild schools.
Individual fish removed from a school will become agitated. Isolation from conspecifics can even cause fish to present a higher respiratory rate. It is much easier for a predator to hunt down and devour a fish swimming alone. The same holds in reverse; fish can protect their territory more efficiently when in groups.
A complex combination of sensory organs enables fish to perform those smooth schooling movements we admire. Perhaps one fish takes an action, say attempts to evade a predator, its neighbors pick up on that movement and soon the whole school responds. Schools turn, contract, expand, even part, and come back together, all without missing a beat. What a phenomenal force they are!
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F.W.H. Beamish, in Fish Physiology, 1978
Stephen C. Pratt, in Encyclopedia of Animal Behavior (Second Edition), 2019
Larsson, in Reference Module in Earth Systems and Environmental Sciences, 2014
Madeleine Beekman, in Encyclopedia of Animal Behavior (Second Edition), 2019
https://daily.jstor.org/how-do-fish-schools-work/
https://twistedsifter.com/2010/07/15-fascinating-schools-of-fish/
