- Grass carp, common carp, Nile tilapia and crucian carp are four predominant species for polyculture rice-fish farming One such system involves four species: grass carp (Ctenopharyngodon idellus),Tilapia sp., common carp (Cyprinus carpio) and crucian carp (carassius auratus), with the first two species making up the majority. The recommended mix is a 24-25 percent composition of both grass carp and tilapia (a total combined composition of 70 percent) with a 15 percent mix of both common and crucian carp (remaining 30 percent) at stocking rate of 2-3 m2. This mix of species can give an optimal yield of both rice and fish