## 1 Introduction  
Annual fish and seafood consumption has been increasing steadily. The amount of fish and seafood obtained globally is limited and annual output has remained at 90 million tonnes for the past decade. For this reason, any increase in the amount of fish and seafood is associated with aquaculture and its dynamic growth. The production of aquaculture accounted for 44% of the total output in 2014 [1, 2]. Installations used for animal production can be classified in regard to the degree of water recirculation: (1) open/flow-through systems, in which water is used once; (2) semi-open/semi-closed systems, in which water is used multiple times before being removed from the system, and (3) closed systems, in which only water loss is replenished. Higher degrees of water recirculation require more complex systems to purify it. RAS denotes technologies of repeated water reuse in a closed system. However, water loss in breeding systems needs to be replenished for multiple reasons, not only because of evaporation. To minimise the need for replenishing water loss, RAS systems are equipped with complex water treatment and purification systems, including mechanical and biological filters, UV sterilisation devices, water ozonation systems and others [3; 4]. The most important of these systems are biological filters, which control toxic nitrogen species and are necessary for RAS operation [3; 4; 5]. The fast increasing of aquaculture production in RAS is not possible without new developed technologies. One of the main focuses is media for biofilters, upon which the effectiveness of nitrification is dependant. This also an influence of biofilter costs, so new and cheaper biofilter media are necessary to involve in further aquaculture production growth [1; 2; 3; 4; 5]. The removal of toxic nitrogen from water is affected by nitrification, consisting of the biological oxidation of ammonium nitrogen to nitrite (III) nitrogen, followed by oxidation of the latter to nitrate (V) nitrogen. A crucial role in the nitrification process is played by Nitro- (oxidation of ammonium to nitrite) and Nitro- (oxidation of nitrite to nitrate) nitrification bacteria [6; 7]. Total ammonium nitrogen (TAN) is one of the major limiting factors in the design and operation of RAS systems [8] and is one of the metabolites which is formed in the digestion of proteins and the transformation of amino acids [9] given to fish with feed. It occurs as a sum of two species: dissociated (ammonium ion, NH\({}_{4}\)\({}^{+}\)) and undissociated (ammonia, NH\({}_{3}\)) [8; 10]. Ammonium nitrogen in its dissociated form is relatively non-toxic, whereas the undissociated form is highly toxic [8; 9; 11; 12; 13; 14; 15; 16; 17]. For this reason, it must be controlled [17] and removed from the system or oxidised to a less toxic nitrogen form [8; 18]. Ammonia levels in RAS systems are controlled by nitrification [17]. The NH\({}_{4}\)\({}^{+}\)/NH\({}_{3}\) ratio depends mainly on the environment's pH. The amount of toxic NH\({}_{3}\) increases along with higher pH, depending on the temperature, pressure and salinity. Excessive amounts of ammonia lead to tremors in fish, coma and death [10]. The ammonia removal process is affected by a number of factors. The most important of them is the availability of oxygen (4.57 g of oxygen is needed to oxidise 1 g of ammonium nitrogen) and the rate of its diffusion into the biomembrane [8]. A decrease in water saturation with oxygen results in inhibition of ammonium nitrogen oxidation [19; 20]. Important factors also include temperature, pH, salinity and organic matter burden on the biological filter [8]. Nitrite nitrogen are intermediate compounds in nitrification [6; 7]. Due to their toxic effect on fish, they are an equally important factor in fish breeding as ammonium nitrogen [7; 14; 21; 22] and are responsible for chronic diseases leading to fish death [7]. Nitrate nitrogen is the last step in the nitrification process [6; 7] and has long been regarded as harmless.