## 4 Discussion
The time in which the ammonium nitrogen concentration was increasing and decreasing differed for the studied filters. The curves of changes in concentration were also different. In the case of filters inoculated with bacterial cultures, the course was similar to that observed in the system, which was restarted. This example supports the suggestion that nitrifying bacterial cultures remain in this system despite disinfection with potassium permanganate. For studies based on nitrifiers immobilized in PVA (polyvinyl alcohol) and then adapted to the salty environment [51], the time needed to remove the elevated TAN concentration (initial concentration of TAN introduced into bioreactors was 10 mg/dm3) to nitrite ranged from 22 days (salinity 30 ppt)to 26 days (salinity 7.5 ppt). These are longer periods than obtained during the author's experiment. In 2015, [PERSON] et al. [52] conducted research on the removal of nitrogen compounds from tilapia culture (_Oreochromis niloticus_) using aquaponic crops (tomato _Lycopersicon esculentum_ and pak choi _Brassica campestris L. subsp. chinensis_). In these studies, they used root systems of cultured plants as growth surfaces for nitrifying bacteria. During the study, the highest TAN concentrations were reached when the TAN concentrations peaked around day 7 and were around 25 mg/dm\({}^{3}\) (tomato) and 32.5 mg/dm\({}^{3}\) (pak choi). These concentrations are higher than those observed in the discussed experiment; moreover, they did not decrease to a similar degree in a comparable period of time. This difference is probably due to the smaller root surface area in relation to the effective surface of the applied biological filter medias. According to [PERSON] et al. [53], the increase in nitrite concentration is delayed in relation to the increase in ammonium concentration (Figures 4, 5, 7 and 8; Tables 1, 3 and 6a). In the conducted experiment, an increase in nitrite concentrations was observed from the beginning of the experiment (Figures 5 and 8). Initially, these concentrations were relatively low compared to the observed concentrations of ammonium nitrogen, however, since the growth curve is exponential, the concentration of nitrite increased very quickly (Table 6a). Subsequently, the increase in nitrite concentration slowed down and slightly collapsed, followed by a reversal of the trend observed in the first period. Nitrite concentrations in all RAS systems dropped rapidly, reaching a relatively constant level (Figures 5 and 8, Table 6a,b). This course of concentration changes was observed in all RAS systems except the system in which the failure occurred (Table 6a). In this system, the period of deceleration and collapse of the upward trend does not occur (Table 6b). Maximum concentration values were reached much faster and they began to decrease faster. This is reflected in the equations describing the course of nitrification (Table 6a,b). The time needed to stabilize the second phase of nitrification ranged from 15 to 33 days (Figures 5 and 8). This is different from the results obtained by [PERSON] et al. [45]. Despite the fact that measurements were carried out for 28 days, no downward trend was observed. This situation occurred in systems not inoculated with nitrifying bacteria. Different results were obtained in systems inoculated with nitrifiers--no increase in nitrite concentration was observed. In the research conducted by [PERSON] et al. [18], the time needed to stabilize the second phase of nitrification was 35 days, which is longer than the results observed in this article, although similar. Comparable times to those obtained during the experiment and to those obtained by [PERSON] et al. [18] were also obtained by [PERSON] et al. [51] during an experiment with the acclimatization of nitrifiers to saltwater conditions. The acclimatization time of biological filters, and thus the time needed for nitrite oxidation to a safe level, was achieved after 33 days (salinity 7.5 and 15 ppt) and 39 days (salinity 30 ppt).