Fish eat animal plankton, which are known as zooplankton. Plant forms of plankton are called phytoplankton, which exist as food for zooplankton. The most common zooplankton are copepods. They are crustaceans about the size of a grain of rice, according to Oceanic Research Group.
More copepods exist than any other animals on Earth. They live in all the world's oceans and are greater in number than land insects. Other zooplankton types range from sea butterflies to jellyfish, so not all plankton are small. However, all zooplankton are prey for larger sea animals. Scallops, anemones and basket stars also feed on plankton.