## 1 Introduction  
Feed is one of the main factors that influence fish welfare and the microbiological status of water in aquaculture and freshwater ecology [1]. The nutritional value and microbiological quality of feed determine fish weight gains, and the sanitary and epidemiological safety of aquatic organisms and the aquatic environment [2; 3; 4; 5; 6]. Synbiotics containing probiotics and prebiotics enhance the health benefits of feed. They promote the growth and metabolic activity of beneficial microorganisms in the host's gastrointestinal tract without compromising endogenous gut microbiota [7; 8; 9]. Probiotics are natural microbiome bacteria that deliver multidirectional beneficial effects for living organisms (humans and animals) at the local and systemic level [2; 10]. The role of probiotic feed microbiota in the maintenance of gut homeostasis is increasingly recognized as a critical success factor in fish breeding [6; 11; 12; 13]. The group of probiotic bacteria includes members of the genus _Carnobacterium_[14; 15; 16; 17]. _Carnobacterium malatomaticum_, which colonizes natural aquatic habitats, is one of the most metabolically active probiotic bacteria in the digestive tract of animals. This bacterial species easily adapts to changes in habitat conditions such as temperature, salinity, and pH, and it delivers health benefits for the host organism [14; 18; 19; 20]_Carnobacterium malatomaticum_ effectively inhibits the development of pathogenic bacteria and is regarded as a potent immune stimulator in fish [14; 21; 22; 23]. The growth and activity of probiotic bacteria are influenced by environmental conditions that can be optimized with the use of prebiotics [24; 25]. Prebiotics are non-digestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of bacteria colonizing the gastrointestinal tract [7]. Animal and plant meals are one of the main ingredients of fish feeds [26]. However, plant meals contain anti-nutritional factors, and their applicability in compound fish feeds is limited [27]. One of exceptions is amaranth meal, characterized by a low content of anti-nutritional factors, mainly saponins and phytic acid [28]. Amaranth meal contains lignins and various compounds with antioxidant, antibacterial, antiviral, and fungistatic properties [29; 30; 31]. Amaranth seeds are also abundant in other health-promoting substances, such as squalene and fiber [32; 33]. In a study by [PERSON] et al. [34], feed supplemented with 20% of amaranth meal promoted the growth of rainbow trout (_Oncorhynchus myskiss_) and improved the digestibility of dietary nutrients. A microbiological analysis in a pilot study conducted by [PERSON] and [PERSON] [35] revealed that amaranth supplementation can prevent excessive growth and proliferation of _Staphylococcus_ sp. bacteria, _Clostridium_ sp. anaerobic spore-forming bacteria, yeasts, and molds in compound fish feeds. A similar beneficial influence of amaranth meal on selected probiotic strains was also observed by [PERSON] et al. [36] who demonstrated that amaranth meal stimulated the fermentation ability of ten probiotic strains (_Lactobacillus_ spp. and _Bifidobacterium_ spp.) The microbiological composition of fish feeds significantly influences fish health and weight gains. This parameter is particularly important if feeds contain harmful microorganisms that compromise fish health, disrupt digestive metabolism, and compromise the reproduction and survival of farmed fish [4]. Feeds should be characterized by microbiological stability and high quality to guarantee the optimal health status and physiological condition of farmed fish. Nevertheless, not all undesirable microorganisms are eliminated during fish feed production. According to the literature [4; 37], the standard extrusion process does not guarantee complete elimination of various microorganisms from fish feeds.