They are part of the structure of every fish cell, they are made of amino acids, which in turn are necessary for fish in virtually all chemical, biological and metabolic processes.
According to the type of bond, we have unsaturated fats (double bonds) and saturated fats (single bonds). Just as in the case of humans, the former are desirable for them because of their speed in binding with other compounds (high chemical activity). In fish nutrition, fats primarily play the role of energy material (the same as in humans) and enable the absorption of vitamins soluble in them.
We have simple carbohydrates (e.g. glucose, fructose) and complex ones (e.g. starch, cellulose, chitin). All simple carbohydrates are absorbed without prior chemical treatment. Complex carbohydrates are digested under the influence of appropriate enzymes, but there are also those that are not digested (assimilated) at all, e.g. fiber (cellulose), chitin. for their beneficial effect on digestion. The source of fiber are the outer layers of cereal grains (hulls, shells), oat flakes, legumes, brassica vegetables.
They are an essential component of food, because the fish's body is unable to produce them, and their lack leads to a serious disease - avitaminosis. Vitamins perform regulatory and catalytic functions for most physiological processes. We can divide them into fat-soluble vitamins (A, D, E, K) and water-soluble vitamins (C, B vitamins).
Substances that are found in larger amounts in the fish's body (calcium, phosphorus, iron, magnesium, sodium, potassium and chlorine) and microelements (trace elements) - those that are much less (zinc, copper, manganese, cobalt, tin, iodine, fluorine, molybdenum). In the body of fish, these substances perform various functions, most often they are components of enzymes, hormones, they are responsible for the acid-base and osmotic balance, they are necessary for the production of blood dye, etc.
Herbivorous fish should consume 75-85% of plant-derived foods in their diet, and in highly specialized herbivores this share should be even 90%.
Carnivorous fish should consume 60-75% of food of animal origin in their diet, with typical predators even 75-85%.
In omnivorous fish, these shares are 50% to 50% for plant and animal food