Fish have very complex jaws so I’m going to try to explain how they eat.
Fish have a retractable buccal cavity that lets them suck prey in from a short distance. Fish use a vacuum like force to eat prey. They suck in their food and open up their gills to let the water out of their mouth creating a vacuum. Operculum flaps cover the gills so water doesn’t flow into the gills, but they open it to get water out.