Fish have a basic digestive system with stomachs that are adapted to the type of food the fish eats. Food that is taken in is partially digested in the stomach before moving into the intestine where it is further digested, and it is in the stomach that the nutrients from the food eaten are absorbed.
Food that is not absorbed exits the fish's body through the anal opening as waste. It is important to feed fish the food that is recommended for them. For example, herbivorous fish require a high-fiber diet, while carnivorous fish need protein-rich food. Carnivores have shorter intestines and overall more simplistic systems, thus the process of moving food through the body differs slightly.