Anemia in fish can be easily recognized by examining the gill color and physical properties of sampled blood. The number of erythrocytes is generally within 1-3 ${ imes}10^6$. Hematocrit (Ht) reportedly ranges 35-45% in many of reports which is nearly the same as that in mammals. Anemia in fish can also be classified as hemorrhagic, hemolytic and hypoplastic ones. Fish are also expected to have similar pathological effects by anemia as in mammals. From the fact that fish can survive under extremely low hematocrit below 10% and antarctic icefish (Chanenocephalus aceratus) has no erythrocytes, different pathological effects were expected. Anemia is considered to have one of the most important health parameters, based on the pathological aspect. Therefore, by reviewing clinical and histopathological findings of most of fish diseases already known and then adding our results of experimental anemia, diseases and factors related to anemia were summarized and the pathological effects was discussed.