1.Bullying from other fish and lack of hiding spaces:
Create suitable hiding spaces for every fish in the tank, otherwise, there are going to be fish that are stressed and bullied. Unlike their wild environment, these fish are confined and cannot get away from aggressors. Aggression is a very real problem in many tanks that leads to many injuries, infections, and death.
Do not overstock your tanks. Overstocking of the tank is a common problem that contributes to almost all of the stresses in the above list, from water pollution to oxygen depletion to harassment. If you want to stress your fish, put too many in the tank and it will happen every time.
3. Keeping lone schooling fish
If you do not have a large enough tank to keep an entire school of fish, stick to fish that do fine as solitary fish or in pairs or trios. Keeping a schooling fish alone is not a good idea. These fish will tend to develop depression and may stop eating and swimming enough to stay healthy when kept by their lonesome. They also tend to express aggressive behavior towards their tank mates whereas they are actually peaceful in their group.
4. Water Treatment
Adding stuff to the water to treat a disease or water condition can be stressful to your fish. Try to avoid treating the water if at all possible and always use a quarantine or treatment tank. Copper is an excellent treatment for ich or velvet, but it can be toxic and stressful to fish. Of course, using it is much better than letting a fish die from velvet, but it should never be used in a tank with healthy fish.
Improper nutrition is also a commonly overlooked stressor of fish. Many fish can live on minimal nutrition with old or stale flake foods, but this poor nutrition is a chronic stress. A variety of well-preserved dry foods as well as freeze-dried, fresh, and frozen foods specifically designed for individual species are necessary to prevent chronic nutritional stress.
6. Disturbance of the tank
Disturbing the tank through banging on the glass, constantly netting fish, or rearranging decor stresses fish. This should be kept to a necessary minimum.