In some of the countries the main purpose of its introduction, in addition to culture, is biological aquatic weed control in natural waterways, lakes and man-made lakes. Grass carp, like other cyprinids, has a toothless mouth but has specialized pharyngeal teeth for rasping aquatic vegetation. These teeth occur in two rows, the upper consisting of two small teeth on either side and the lower of strong comb or file-like teeth comprising four on the right and five on the left pharyngeal bone. In fishes of total length 30 cm and below, the lower pharyngeal teeth have a serrated cutting surface, while in larger fish the teeth are thicker and have double flattened serrated cutting and rasping surfaces. These patterns of teeth structure are associated with the change in the feeding habit.
Large fishes are able to masticate the leaves of tough land plants such as fibrous grasses.
Digestion in the grass carp is said to be incomplete and about half the food material ingested is excreted as feces which, it is believed, can support directly or indirectly a large biomass of other species of fish. Cladocera, copepods and benthic algae form the diet of fry 13-17 mm long to which organic detritus is added up to a length of about 23 mm. As the fry grow to about 30 mm length, phytoplankton and minute algae become conspicuous additions to the diet. For fish above 30 mm, the natural diet is virtually exclusively macrovegetation, tender aquatic weeds like Wolffia initially and a wide variety of other aquatic macrovegetation and even softer land plants as the fish grows. The fry and larger fish take to substances like cereal brans, oilcakes, silkworm pupae, kitchen refuse, night soil and dung which are often given as supplementary food.