Phytoplankton diversity of a sewage fed fish pond was studied throughout the year in relation to seasonal abundance and nutritional parameters. A large number of cyanophycean and chlorophycean genera appeared at different season and directly served as feed source of the zooplankton and the fishes in one hand, and played an important role in reducing the nutrient concentration and pollution level of the Municipal waste of Calcutta Metropolitan, on the other. Dominant growth of Spirulina platensis was observed in the month of summer, beside this Scenedesmus, Chlorococcum and many other genera appeared in different season. As a result, the phosphate level was reduced from 40.2 mg/L of raw sewage to 2.0-5.0 mg/L of the fish pond and high amount of ammonia was oxidized to nitrate. Similarly the BOD level was reduced by twenty times, but the DO and the chlorophyll content increased in the fish pond, therefore, creating a suitable condition for fish cultivation. Copyright © Enviromedia.