This is one of ten eBooks on the commercial production of ornamental fish. It is based predominantly on the author's 30 years of experience, mostly on his own farms. While a few of the eBooks are purely commercial, others, particularly those on breeding, will also be helpful to hobbyists, understandable even by beginners. The following description is provided as a guide. All about pathogenic disease, with some golden rules on health management on fish farms, and descriptions and illustrations of the causative organisms of many common diseases, with treatments. Includes identifying signs and symptoms, and common causes of ill health and disease in fish. Predominantly commercially oriented, but potentially of interest to serious hobbyists.