A fish can get stressed out for many reasons. Aggressive tank mates, transporting, too much water current caused by power heads, poor water quality, disease, too much light, hitting the glass, lack of privacy in a transparent tank etc. are common causes of fish stress.
Stress directly affects the normal functioning of a fish. It's mental and chemical balance is affected making it prone to disease as the immune system gets weak. After prolonged exposure to stressful situations, a fish can develop chronic conditions, organs may malfunction or fail causing imminent death.
One example of fish stress is keeping 2 male convict cichlids together in one tank. They are territorial and aggressive in nature and will fight each other from time to time. In the wild, the loser runs away never to be seen again but in a fish tank, such a thing is not possible. This causes stress of the very presence of the dominant male. These fish may display signs of fear by sticking close to the surface of the water at the corners. Appetite may be affected too.
So please keep in mind the basic idea about fish stress while keeping fish and give them the best experience possible to make them happy and healthy.
The Convict Cichlid (Kindle Edition Ebook)
The Convict Cichlid (Kindle Edition Ebook)