Nutrition in aquaculture is critical, because feed represents around 40- 50 % of the production cost. To achieve a balanced feed trying to use quality raw material at the lowest possible cost is a target.
A good balanced fish feed must contain the needed nutrients in the absolute quantity and maintaining the proportion (protein (18-50%), lipid (10-25%), carbohydrate (15-20%), ash (< 8.5%), phosphorus (< 1.5%), and water (< 10%), and trace amounts of vitamins, and minerals) of the following ingredients, depending on the target consumer and exploitation system:
Protein is the most expensive part of the formulation so it must be accurately determined. Proteins are composed of carbon (50%), nitrogen (16%), oxygen (21.5%), and hydrogen (6.5%). Fish are capable of using a high protein diet, but as much as 65% of the protein may be lost to the environment.
The 10 essential amino acids that must be supplied by the diet are: methionine, arginine, threonine, tryptophan, histidine, isoleucine, lysine, leucine, valine and phenylalanine. Of these, lysine and methionine are often the first limiting amino acids.

              
     FIG I: LYSINE                             FIG II: METHIONINE

At the same time, different parametres have direct relation with the required quantity of protein:
• The type of animal: the carnivorous ones have higher requirements; while herbivorous have the lower ones. Protein levels in aquaculture feeds generally average 18-20% for marine shrimp, 28-32% for catfish, 32-38% for tilapia, 38-42% for hybrid striped bass
• The kind of exploitation: high density exploitations have higher demand than low density aquaculture systems.
• Age of the fish: young fishes need more protein than the elder ones.
• Water temperature: as the temperature increases the protein demand decreases.
• Water quality

As said at the beginning, the relation between quantity of protein, fats and carbohydrates must be adequate, if not fish will use protein for energy and life support rather than growth.  On the other hand, an excess of energy related to protein content on the diet will result in fat deposition.