Fish may begin to waste away and that is a common sign of intestinal parasites, or fish tuberculosis.
Fish Tuberculosis or TB Mycobacterial Infections Symptoms and Prevention
Fish tuberculosis is a mycobacterial infection where the fish ingests the organism via cannibalism and then stores it. The tuberculosis is dormant until the fish undergoes considerable stress. Those forms of stress are identified.
So I made a video that discusses the tuberculosis issue in fish, in detail.