## 4 Discussion
Longer nitrite elimination times than observed during the experiment were also reported by [PERSON] et al. [52]. The time needed for oxidation of nitrite to nitrate in aquaponic cultivation was about 40 and 50 days for tomato and pak choi respectively. As in the case of ammonia oxidation, the surface of the roots on which nitrifying bacteria developed was smaller than in the tested biological filter medias. For nitrates, the recorded concentration values increased from the very beginning of the experiment. Initially, as with nitrites, this increase was slow to eventually accelerate. After a period of dynamic growth, nitrate concentrations stabilized (Figures 6 and 9; Table 7). For CF, despite the similar nature of the increase curve, the concentrations were higher during the first part of the experiment. No similar trend was observed for the same type of filter in the second part of the experiment (Figures 6 and 9). Higher values in the initial phase of growth in the second part of the experiment show the concentrations in the RAS system in which the failure occurred (Figure 9). The similar nature of the increase in nitrate concentration was reported by both [PERSON] et al. [45] (circulation vaccinated with nitrifying bacteria) and [PERSON] et al. [18]. [PERSON] et al. [45] did not observe an increase in the concentration of nitrates in the RAS system inoculated with nitrification, which was associated with the incomplete nitrification process. The nitrate concentration increase curve presented by [PERSON] et al. [51] was comparable to the present experiment. At the end of the experiment, a decrease in nitrate concentration was observed in individual RAS systems. Probably this decrease was associated with the development of heterotrophic bacteria in the volume of water. During this experiment, it was analysed how the concentrations of nitrogen forms in water are shaped in experimental RAS systems. On this basis, the course of the nitrification process was determined along with the time needed for the biological filter to mature. The maturation of the biological filter is largely dependent on the temperature at which the process takes place (Table 8). In systems using cool water, a mature filter can be obtained after a few months of its work, while this period is significantly shorter in systems developed for the needs of thermophilic species. Another factor that may affect the maturation of biological filters is the presence of the desired nitriifying bacteria. Biological filters inoculated with nitriifying bacteria, operating at 25 \({}^{\circ}\)C, show desirable properties after 53 days [54]. The use of water from tilapia culture (temperature 26.4 \({}^{\circ}\)C) allowed a functional biological filter to be obtained after 56 days [5]. These results do not reflect the information provided by [PERSON] [55], who states that at 18 \({}^{\circ}\)C, it takes 40 to 60 days to obtain a working biological filter. In addition, the periods given differ from those obtained by [PERSON] et al. [18]. Studies have shown that the time needed to mature a biological filter without inoculating the culture cycle with nitriifying bacteria cultures is 35 days at 23 \({}^{\circ}\)C.  
The conducted tests showed that at 25 \({}^{\circ}\)C, without introducing nitriifying bacteria cultures into the RAS system, the time needed for the filter to mature is from 21 to 33 days. If nitriifying bacteria are present, this period may be shorter. In one of the RAS systems used for the experiment, a failure occurred. As a result of the failure, it was necessary to restart the RAS. This RAS was dried and disinfected in accordance with the adopted procedure. In this system (drip filter, CF), a functional biological filter was obtained on day 16.