Abstract: The invention relates to water regeneration in a fish breeding complex combining closed and flow-through water supply systems. A module of a biological filter in a water regeneration system comprises a reservoir, an aerator, a channel for sludge accumulation and discharge and a reservoir bottom sloped in direction of the water movement and, with the water surface, forming a diffuser providing for circulation of water and filler in the biological filter. Fish breeding complex comprises fish breeding pools and a water regeneration system comprising a mud settler-denitrificator, a device for water lifting and aeration, a biological filter, degassing and disinfection units. Each fish breeding pool comprises water oxygenation and disinfection systems, dosage units, water discharge systems and insoluble residues collecting and discharge systems. In the water flow-through mode, water regeneration system is switched off and water supply from an outside source and wastewater discharge are switched on.