## 4 Discussion  
In the experiment, the dynamics of changes in nitrogen compounds in the water used for rearing fish in semi-closed RAS were analysed in detail. The dynamics of nitrogen compounds (ammonium, nitrite, nitrate) during the maturation of biological filters as well as during the operation of mature filters were examined. Furthermore, the usefulness of PPA and PSC as fillings for biological filters was demonstrated. Due to the differences in construction, all three fillings have their own advantages and disadvantages. Therefore, when comparing fillings, the analysis was based on the analysis of changes in concentrations of nitrogen compounds in water and the time needed to achieve full filter functionality. Comparing structural parameters, especially similar in terms of the size of the active surface of PSC and PPA fillings, could be subjective. The impact of structural parameters should be examined at a later stage of the study, in which the impact of individual factors (e.g., oxygen, temperature, suspense solids, diffusion rate, etc.) would be analysed on the dynamics of the nitrification process. Water exchange is necessary in aquaculture farms using a closed water cycle equipped with biological filters without denitrification due to ever increasing level of nitrate. Denitrification is the process of reducing nitrates to gaseous nitrogen. It is an anaerobic process that can sometimes occur in biological filters overloaded by biogas. The amount of water needed varies depending on several factors. The most important is the increase in nitrate concentration, which in high concentrations shows adverse effects on aquatic organisms [21; 23; 33]. At the start-up stage of RAS facilities, water change is required for a similar reason. High concentrations of toxic forms of nitrogen occurring during the maturation of biological filters require the use of water changes not only to supplement evaporation. In the experiment, a one-time top-up of 150 dm\({}^{3}\) daily was used. This allowed for the dilution of nitrogen compounds and while supplementing calcium compounds in water, it also allowed better observation of the dynamics of nitrogen compound transformation processes. An important factor was also the use of common carp--a model species which is considered to be relatively resistant to adverse environmental rearing conditions [34]. Mechanical and biological filters must work together for the proper functioning of aquatic organisms [7; 26; 27; 35; 36]. The importance of the filtration process becomes significant when the breeding facility is built in RAS technology [22; 37; 38; 39; 40; 41; 42]. For the proper course of biological purification in the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Filter,** & **Regress Equation** & **Correlation** \\
**Filling** & & **Coefficients (R)** \\ \hline Submerged, & \(\text{y}=-0.0005\text{x}^{3}+0.0392\text{x}^{2}-0.1142\text{x}+0.135\) & 0.9844 \\ Supmerged, & \(\text{y}=-0.0005\text{x}^{3}+0.0425\text{x}^{2}-0.3631\text{x}+1.5747\) & 0.9946 \\ PSC & \(\text{y}=-0.0006\text{x}^{3}+0.0367\text{x}^{2}+0.5992\text{x}-2.1153\) & 0.9797 \\ Clapping, & \(\text{y}=-0.0017\text{x}^{3}+0.1118\text{x}^{2}-1.1623\text{x}+3.931\) & 0.9884 \\ PIPing, & \(\text{y}=-0.0015\text{x}^{3}+0.1082\text{x}^{2}-1.3589\text{x}+4.808\) & 0.9882 \\ PSC & \(\text{y}=-0.0005\text{x}^{3}+0.0174\text{x}^{2}+0.9072\text{x}-1.355\) & 0.9968 \\ CP & \(\text{y}=-0.0015\text{x}^{3}+0.1052\text{x}^{2}-1.1256\text{x}+3.7311\) & 0.9926 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Nitrate nitrogen concentration analysis in each experimental system using regress equation and correlation coefficients (R): PPA—polypropylene aggregate, PSC—polyethylene screw caps for PET bottles, CF—commercial fittings HXF12 KLL, CFsf—control submerged filter for dripping filters, commercial fittings HXF12 KLL. nitirification process, the cooperation of two groups of microorganisms is required.