An experiment was conducted to know the different species of phytoplankton that silver carp (Hypophthalmichthys molitrix) and Nile tilapia (Oreochromis niloticus) feed on. This experiment conducted in the Central Laboratory for Aquaculture Research (CLAR) Abbassa- Sharkia Governorate- Egypt using 6 concrete ponds. The experimental extended for 4 months from September to desember (2011). The ponds was fertilized by inorganic fertilizers. Fish samples were collected monthly, individual weight and length were measured. Gut content samples were taken and microscopically examined. The results showed that total alkalinity (T.Alk.) and total hardness were significantly increased in the first treatment (Nile tilapia). Nitrate concentration was significantly increased in the third treatment (Silver carp). Chlorophyll "a" concentration was significantly increased in the second treatment. The obtained results indicated that cyanobacteria representing 72.3% of O.niloticus gut content, while green algae representing 69.9% of silver carp.