Lymphocystis

The most common viral disease of tropical fish is lymphocystis disease, caused by an iridovirus that infects dermal fibroblasts of the fish enlarging the fibroblasts by up to 100,000 times their normal size, making them visible to the naked eye.  White nodules on the skin and fins represent clusters of hypertrophied cells.  Histopathology or examination of a scraping microscopically confirms the diagnosis.  This disease rarely causes mortality and there is no effective treatment, but improved environmental and nutritional conditions may cause the lesions to spontaneously regress.