Zooplankton are small animals that float freely in the water column of lakes, reservoirs, ponds and oceans and whose distribution is determined by water currents and mixing. The zooplankton community of most lakes ranges in a size from a few tens of microns (Protozoa) to more than 2 mm (Macrozoo plankton). In terms of productivity, the dominant groups of zooplankton in most lakes are Crustacea and Rotifera. Zooplankton plays an important role in aquatic food webs because they are important food for fish and invertebrate predators and they graze heavily on algae, bacteria, protozoa and other invertebrates. Zooplankton communities are typically diverse (more than 20 species) and occur in almost all lakes and ponds. Zooplanktons are rarely important in rivers and streams because they cannot maintain positive net growth rates in the face of downstream losses. Zooplanktons are small floating or weakly swimming organisms. They are very important as primary consumers. They are important food base for secondary consumers including fish. Rotifers are an important part of the freshwater zooplankton, being a major food source. They are filter-feeding with corona. Most of them are littoral, sessile, but some are completely planktonic. They are too small to be important as food for most fish. They may be important in diets of some larval fish. Most rotifers are around 0.1 - 0.5 mm long. Cladocerans are small crustaceans (0.2-3.0 mm) with head and body. Body is covered by bivalve carapace. They swim by using large 2nd antennae. Cladocerans are large species favored by many fish (visual and filter-feeders). More energy returns from bigger species. Zooplanktons are preferred natural food for larval stage of fish and prawn.