The main indicators that highlight the sanitary state of the biofilter in the installation of water supply (RAS) for growing rainbow trout are the number of microorganisms in the water of the reactor and the state of the biofilm of the filler.

The results of the indicators used in the evaluation of the process of biological water purification from organic substances, which serve as a source of nutrition for the micro flora, eliminate toxic substances in water, the process of destruction of the pathogenic micro flora due to antagonism, competition in metabolic processes, etc. are presented. On a number of indicators of the state of the biofilm of the filler, the producers pay less attention, although the work of the whole ultrasound and the state of health of the fish depends on these indicators. Permanent micro biocenosis of  biofilter in industrial trout farms is MAPMM. The density of the microbial bio film, which was formed on the bio filter fillers (static claydite, RK PLAST, AQ-25, KALDNER K1P).

The need for economical use of water encourages the development of methods for efficient water use, including reuse or circulation of water. That is why the ability to work perfectly with closed water supply systems when cultivating valuable fish species is worth exploring and deep understanding of all the mechanisms of water filtration. Given that mechanical processes can be controlled and controlled, biological filtration systems function on the interaction of microorganisms with each other and with the environment. A separate definition assumes compliance with the maintenance of physical and chemical parameters of water of closed systems, the deepening of research on biotic and abiotic parameters is necessary in order to improve the quality of filtration of water used in the agricultural sector. The physiological development of rainbow trout at all its stages in industrial trout farms depends on the state of the bio filter microbiocenosis. A key unit of a biofilter is a reactor where a filler is placed, which is designed to increase the contact surface and ensure the growth of bacteria [4, 5]. In the process of operation of closed water supply plants (RAS), the surface of the loading material (filler) overgrown with a bio film formed by colonies of aerobic microorganisms. Many scientists who study the peculiarities of growing fish in industrial farms indicate the important role of micro flora in sanitary practice.

The aim of the work was to show quantitative changes in the dynamics of mesophilic aerobic and facultative anaerobic microorganisms (MAPANM) and psychrotrophic microorganisms (PsmM) in water of a biofilter reactor for use of different types of filler, as well as to determine the density of microbial biofilms formed on the fillers of the biofilter.

Conclusions and perspectives of further research.

Permanent micro biocenosis of bio filter in industrial trout farms is MAPAnM. Their amount in the water of the biofilter reactor since the beginning of its formation in the micro flora increased and on the 30th day it turned out to be higher than the use of expanded clay in 172 times, RK PLAST - 1019, AQ-25 - in 1118 and KALDNER K1P – 1280 times. The number of psychrotrophic microflora on the 30th day of the colonization of the biofilter has increased, compared with the beginning of the experiment, for 3625 times as a keramzite filler, RK PLAST – by 14838709,6 times, AQ-25 – by 4000000 times and KALDNER K1P by 3379 ,3 times. The density of the microbial biofilm, which was formed on the biofilter fillers, increased during the whole period of the experiment. The densest biofilm was formed by microorganisms on the RK PLAST filler for 21-25 days – 2,76 ± 0,07 units. and for 26-30 days - 2,94 ± 0,08.

Key words: rainbow trout, biofiltration, microbiocenosis, mesophilic aerobic, facultative anaerobic microorganisms, biofilter fillers, nitrification bacteria.