## 3 Results
### Statistical Analysis of Results  
The results do not have a normal distribution (\(p<0.05\), Shapiro-Wilk test). A statistical analysis of the results using a Kruskal-Wallis ANOVA rank test revealed significant differences between the RAS systems (\(p<0.05\)). Detailed results are provided in the table (Table 5). Analysis of the results of ammonium concentrations in individual RAS allowed the determination of two periods of a different character for which regression equations take a different form. After placing the fish, the concentration of ammonium in individual systems increased. High levels of ammonium were noted from day 3, followed by low ones. There was one day of intermediate values between periods. Increases and decreases of ammonium concentration occurred rapidly. In the case of a system in \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Submerged Filter**} & \multicolumn{3}{c}{**Dripping Filter**} \\ \hline  & PPA & PSC & CF & PPA & PSC & CF & CFsf \\ \hline Ammonium & a & b & a & bc & c & bc & ab \\ Nitrite & bcd & bcd & a & cd & d & abc & ab \\ Nitrate & bc & bc & a & b & c & ab & bc \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Analysis of concentrations of nitrogen compounds using Kruskal-Wallis ANOVA test on ranks (\(p>0.05\)) showed differences between individual RAS systems. Results of a multiple comparison test of mean ranks for all samples: 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. Data marked with the same letter in rows did not differ statistically. Figure 9: Nitrate nitrogen concentration in experimental systems using dripping biological filters: 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. which a failure occurred and it became necessary to restart it instead of a period with high concentrations, there was a period during which the concentration gradually decreased (Tables 1 and 3). The analysis of nitrite concentration resulting in individual RAS allowed the determination of four periods with a different nature of the course. These periods for different RAS systems have a different duration but show a similar nature. In the first period, the regression equation describing the increase in concentration takes the form of an exponential function. In the second period, the concentration initially rose, reached the maximum level and began to decline and the equation takes a polynomial form. In the third period, characterized by a decrease in concentration, the equation takes on a power form. In the fourth period, nitrite concentrations remain at the same level without significant decreases or increases, and the equation again takes on a polynomial form (Table 6a,b). The concentration of nitrates in RAS systems show a similar tendency. After the initial slow increase in concentration, accelerated growth followed and was then inhibited. An analysis of the obtained results showed a decrease in nitrate concentration at the end of the experiment, which is reflected in the derived regression equations (Table 7).