Stress is any condition that causes physical or mental discomfort that result in the release of stress-related hormones or results in specific physiological responses. Stress is present in the lives of all living things and is the force that brings about physical change and adjustment. The nature of keeping aquatic species in confined environments generates many stresses that are unique to aquarium fish. This study explains the types of environmental stress observed in fish. Some of these types include social stress due to high density of population in intensive aquaculture, physical stress caused by changes in temperature, oxygen and pH, chemical stress due to exogenous and endogenous contaminants, physical stress due to trauma, and nutritional stress caused by deficiencies or excess of some nutrients in the diet. It also describes the latest methods to diagnose them. Among these methods one can include molecular and biochemical indicators which measure DNA adductors, measurement of steroid hormone receptors, measurement of metallothionin and lipid peroxidation. A review of the hormonal biochemistry that registers and measures the secondary endocrine responses to stress like catecholamine, plasmatic cortisol, propiomelanocortine and plasmatic glucose is also described. Other methods to measure the heat induced stress is by means of the heat-shock proteins and reproductive parameters. There is also a computerized noninvasive method to measure stress in fish. However, the stress that affects fish is different and much more widespread. Small amounts of stress can be harmless or even beneficial, but high levels of stress or prolonged periods of stress can create severe health problems. Finally, the histopathological methodology based on the observation of rodlet cells, and the modifications on size and form of some cells is another measuring method.
