Abstract: Aquatic animals that emit electrical signals are used to detect changes in the operating conditions of an aqueous medium (e.g., the presence of a pollutant). In one embodiment, the aquatic test animals are fishes of a species known as Apteronotus albifrons, whose electrical signals have frequency variations on the order of 0.1% under constant operating conditions; the temperature of the aqueous medium is stabilized to within 0.1.degree. C.; each aquatic test animal is placed in a perforated refuge within a holding tank in which the aqueous medium flows in a vertical direction; and three or more aquatic test animals are used, wherein phases of nychthemeral activity of the aquatic test animals are artificially offset to achieve overlapping of the nychthemeral activity of the aquatic test animals. All of these combine to yield a highly sensitive and reliable system for real-time biological monitoring of physico-chemical parameters of an aqueous medium.