Minimizing and eliminating sources of stress increases the chances of keeping tank inhabitants healthy. The exact amount of stress an individual fish can take depends greatly on what species it is, its age and size, etc. A stressed fish is a weakened fish. Although it may appear healthy to the casual observer, it will be more susceptible to disease, injury, etc. Some fish tend to always stay near the top of the water, others near the bottom.
Some fish swim continuously, others stay in one place.
Deviation from their norm usually indicates stress. Possible causes include low oxygen concentration due to poor water circulation, toxins that have damaged its gills, high ammonia or nitrite levels, etc. Fish won't eat, or doesn't eat as aggressively as in past. Fish stays hidden continuously and won't come out where it can be seen. Fish has nicked fins, open wounds that don't seem to heal.