Some fish do eat seaweed. They are called herbivores. The most common type of food that fish eat, however, is plankton.
Fish eat a variety of different things in the ocean. For some this includes seaweed. An example of a fish that eats seaweed is the marble parrotfish. It is found in tropical waters and eats things like algae and seagrass. Other fish that eat seaweed include pin fish, thee kings island butterfish, surgeonfish and rabbitfish
Not only do fish eat plants, in some circumstances this diet is essential to the ecosystems of the world's oceans. For example, research published in a journal called the Proceedings of the National Academy of Sciences found that herbivorous fish where critical to the prospect of restoring coral reefs. This is because seaweed competes with coral for the natural resources on the ocean floor. Plant eating fish remove that competition from the coral, allowing it to grow and thrive.
Other things that are included in the diets of fish include plankton, both zooplankton and phytoplankton. Zooplankton are animal plankton, while phytoplankton is made up of plants.
Other animals in the ocean also eat plants. This includes turtles, manatees, dugongs, sea urchins and sails. Some other marine invertebrates are also herbivores.