Most small fish eat small crustaceans and other small animals, not algae.

Most phytoplankton, i.e. unicellular algae, is eaten by zooplankton, i.e. mostly by small crustaceans, like copepods.

The fish that eat other animals vastly outnumber the fish that eat algae or aquatic plants.

In open water environments, the kinds of bigger animals, i.e. fish, cephalopods, chaetognaths, cnidarians, ctenophores are almost all predators who, unless they eat each other, they are eating zooplankton that consists mostly of small crustaceans and of planktonic larvae of the animals that live on the bottom.