All fish are heterotrophic. Beyond that self-evident statement, few generalizations can be made.
Fish feed on almost every type of food.
Esophagus very distensible muscular tube. Anything which fits through the buccal, pharyngeal apparatus will fit easily through the esophagus, even if it doubles on itself. Stomach fish-shaped sack in many predators. Absent in many plankton feeders, herbivores, and parasites. Typically better developed in more carnivorous species.