The Phylum protozoa brings together several organisms evolutionarily different that may act as ecto or endoparasites of fishes and other aquatic animals over the world being responsible for diseases, which, in turn, may lead to economic, social and health impacts in many countries. Though ectoparasitic protozoans affect a wide variety of fish species and pose tremendous impact on the economy and health of people worldwide, only little attention is given towards the disease. Therefore, this review highlights on the most important protozoan ectoparasites infecting fish and other aquatic animals: with particular emphasis on morphological features, distribution, epidemiology, pathology, pathogenic effects, clinical importance, diagnosis, and prevention and control.