This bachelor thesis describes stressors found in the aquatic environment and how fish respond to them. At the beginning, there are listed all types of stressors: chemical, biological, physical and procedural, as well as the consequences of that stressors. In the rest of thesis, the reactions of fish to stress are discussed, and they are divided into 3 responses how fish respond to stress. First, primary response to stress occurs when the fish is exposed to stress, the characteristics of this phase are the excretion of catecholamines (adrenaline and noradrenaline) and cortisol. The secondary response occurs after an increase in catecholamines and cortisol and is characterized by increased heart rate, reduced immune functions, increased flow in the gills, etc. If the organism fails to successfully adapt to stress, a third, tertiary response occurs, during which changes in the functioning of the metabolism occur that are unfavourable for health of the individual. Furthermore, it describes how stress affects fish diseases. Various environmental changes are stressful and reduce the fish's resistance to infections and other diseases.