A composition and method is disclosed for suppressing infections and treating one or more injured or diseased aquatic vertebrate animals, by adding to the water containing the aquatic animal(s) a solution containing the extracted juice and fine particles of walnut husks or the combination of walnut husks and walnut leaves of the walnut trees of the genus Juglans, family Juglandaceae (also referred to herein as "Juglans blend"). Effective amounts of walnut husks or the combination of walnut husks and walnut leaves which inhibit or promote recovery from aquatic bacterial, fungal, protozoan, and dinoflagellate diseases on the surface of or in the skin of the animal or from wounds and abrasions, or to control or eliminate metazoan parasites, or to control or eliminate aquatic single-celled green algae are in the range from 0.035 grams to 0.085 grams of dried walnut husks or walnut husks and walnut leaves prepared as a blend, and added as a standard daily dose per 10 gallons of water until the treatment is completed, which is within several days to several weeks. These walnut husks and leaves come from species of the genus Juglans (species nigra, cinerea, and regia) that are presently grown across the United States and in other temperate regions of the world.