Some planktivores pass ingested algae right through, sometimes still alive. These requirements have only been studied for a few aquaculturally-important species. However, putting the food item through a calorimeter and measuring its energy content will not give an adequate picture of the energy available to a given fish. This depends on the digestibility of the particular food to the particular fish. Carbohydrate can substitute for protein as an energy source, but problems arise with many carnivorous fishes.
Fats are good foods for fish, having a high energy content and being almost completely digestible.
Rancid fats and cottonseed oil are toxic to sallmonids. Minerals are also required, but many can be taken up directly from the water. They are looking at dry weight of food to wet weight of fish produced. Rates of food processing vary from a few hours to a few days, depending on size and kind of food, temperature, and amount eaten.