The general ideas regarding ecological biochemical adaptations in aquatic organisms are based on the results obtained by using a set of field and experimental methods over decades of research on members of marine and freshwater fish fauna and on their food items (mollusks, crustaceans) carried out by Science School in Ecological Biochemistry. Examples of biochemical adaptations in aquatic organisms are provided. Certain conclusions are drawn regarding the common and specific pattern of the metabolic adaptive response in the studied organisms growing and developing under normal conditions and upon changes in the environment. The results improve our understanding of the mechanisms behind ecological biochemical adaptations and their role in maintaining homeostasis in aquatic organisms.