## 4 Discussion
The first group oxidizes ammonia to nitrites and the other group oxidizes the formed nitrites to nitrates [6; 7; 43; 44; 45]. By analysing the concentration of particular forms of nitrogen in the water used during the commissioning of breeding facilities, it is possible to observe individual stages of nitrification and thus determine whether the biological filter has gained functionality [18; 45]. The unpredictability of the maturation process was shown in an experiment carried out by [PERSON] et al. [46] illustrating the effect of the type of biological filter used (fixed and moving bed bioreactors) on nitrification in recirculating aquaculture systems. In that experiment, despite the use of filter fillings operating for six months, the faulty operation of the bed was observed, which was revealed by an initially high concentration of nitrites, whose low level was achieved only after about eleven weeks. According to these authors, such a situation was caused by disturbances in the bacterial composition produced by the shock of transportation to a laboratory. This indicates the exceptional sensibility of nitrifying bacteria to variable environmental conditions. Due to the high concentration of toxic nitrogen compounds, fish rearing in circuits with immature biological filters threatens the loss of fish being reared. This is especially true for salmonids [47]. Therefore, species resistant to elevated concentrations of nitrogen compounds should be used at the start-up stage of aquaculture facilities. The experiment using fish as a source of compounds needed in the nitrification process allowed observation of the work of the biological filter in conditions simulating real fish breeding [18]. Research carried out in this way is very important to properly carry out the maturation process of the biological filter and obtain a filter adapted to the given conditions in RAS systems. A review of the literature has shown that this approach to the topic is rare [18; 48]. Research using synthetic solutions with nitrogen compounds dominates the field [40; 49; 50]. In this experiment, carp was used as a model species [34] with high metabolism [31]. During the experiment, all systems noted a rapid increase in the ammonium content in water to approximately 3 mg N-NH4/dm3 (Figures 4 and 7; Tables 1 and 3). The increase in ammonium concentration resulted from placing the fish in the water cycle and starting feeding them. It should be emphasized that carp is a species with very high metabolism [31]. This level persisted from 4 days (submerged filter, CF) to 9 days (submerged filter, PSC) (Figure 4). In other filters, the duration of elevated ammonium concentration in the circuits was 7-8 days. After this period, there was a rapid, several-fold decrease in ammonium concentration in the RAS. It was assumed that the procedure involving the drying of the circuits and their disinfection through the use of potassium permanganate will eliminate nitrifying bacteria. The abrupt increase in ammonium has proved this assumption. Low concentrations persisted until the end of the experiment. In the system that was restarted, the distribution of ammonium concentrations was different (Figure 7). The first recorded results were about 60% lower than in other systems, and there was a rapid decrease in ammonium concentration. The fall curve was much milder. There is a discrepancy between the results obtained for two systems in which submerged filters filled with CF were used (Figures 4 and 7). The duration of elevated ammonium concentration for these systems was four days and eight days. The results during the experiment are divergent from the experiment carried out by [PERSON] et al. [18], in which no increase in ammonia concentration was observed. [PERSON] et al. [45] reported similar results. In their research, they compared the effects of biological filters that were inoculated with nitrifying bacterial cultures with uninoculated filters. In unvaccinated RAS systems, ammonium nitrogen concentrations quickly rose above 2.5 mg/dm3 and then decreased.