More or less everything in nature, with the rare exception of an adult orca or a fully grown female great white shark will be prey for something else. One consequence of this is that fear will be a prevailing emotional status in nature, as bold and brave animals will be caught and eaten much quicker than scared and cowardly conspecifics. Small fish are potential prey for more predators than almost anything else and will need to be scared and skittish critters.
A predator that is let to close to a small fish will sooner or later catch the fish that do not scare easily and seek refuge when disturbed. On the other hand, small fish cannot be too skittish, as all they then would do would be to hide, which would keep them from foraging and growing. Thus, there is a thin line for prey fish balancing between being to bold and getting lots of food, but at the same time being killed by predators, and being just to scared, essentially starving themselves without good cause. This is the line where prey should be, balancing foraging benefits and risk of being killed by a predator. Prey fish are born skittish, but there are also behavioural fine tunings going on to asses risk more precisely. Thus, disturbances that occur regularly but are not followed by an attack, will soon be ignored. For example, large fish such as puffers, that do not eat small fish, will after a number of uneventful encounters be allowed much closer to small prey fish than piscivorous (fish eating) fish of the same size as the puffers. Obviously, this is something that can be exploited. Small piscivores (fish that eat fish) will use the harmless puffers or burrfish more or less as a Trojan horse, swimming right next to the big harmless fish and using this as a means to get close to small fish.
Once the small fish has let down its guard and accepted the harmless species getting close, the deceitful predator, often a jack or a trumpetfish, hiding at the side of its Trojan horse, will pounce and attack the unsuspecting prey.
As a side comment, researchers at my department in Sweden have exposed juvenile fish to SSRI´s which act as antidepressants. Traces of such antidepressants are found in freshwater bodies, and at levels occurring currently in nature, the exposed fish showed marked changes in behavior, being much less adverse to risk than earlier. Such thrill-seeking by juvenile fish could obviously be problematic for their survival!