Present up-to-date data on biochemical processes caused by microorganisms in the aqua environment, consider the impact of environmental factors on microorganisms, infections and immunity, pathogenic microflora transmitted through water, sanitary and microbiological characteristics and assessment of the quality of wastewater and natural waters, test zones, microbiological bases for purification, self-purification of natural and wastewater, bacterial products and methods for their determination, the role of microorganisms in increasing fish productivity aqua environments, microflora of live fish.