## 2 Materials and Methods
### Water Samples  
Aquaculture wastewater was collected from the aquaculture system at Leiria Polytechnic Institute in Leiria, Portugal, during the winter (February 2025). The entire system is shown in Figure 1. The operation of the aquaponics system relies on continuous water circulation. The cycle initiates in the fish tank (Tank 1), from which water flows into the drum filter (Tank 2) for mechanical filtration. Following this stage, the water enters the biological filter (Tank 3), where nitrifying bacteria convert ammonia into nitrate, rendering the water nutrient-rich for plant use. Next, the water moves to the grow bed, where plants are supported on floating rafts and absorb the available nutrients to support their development. From there, the flow proceeds to the sump reservoir (Tank 5), where a submersible pump returns the treated water to the fish tank, thus completing the cycle. An additional component--a conical sedimentation tank (Tank 6)--was integrated into the system to handle surplus water from the drum filter (Tank 2). This excess water, instead of being wasted,is redirected and reused through a connection between Tank 6 and the sum reservoir (Tank 5). The design takes advantage of gravity to facilitate water movement throughout the system. Water sampling focused on challenges frequently observed by system operators. The aquaculture system houses _Clarias gariepinus_, commonly known as the African catfish, a species widely farmed for its ability to thrive in high-density environments and tolerate low-quality water conditions [28]. Known for its adaptability, this species can withstand challenging factors such as low dissolved oxygen, elevated ammonia levels, and increased salinity [29]. However, a significant drawback of this setup is its rapid digestive process, which results in a higher production of organic waste and, consequently, a decline in water quality. The increased organic matter contributes to intensified water coloration, particularly in the sum reservoir (Tank 5), where the water took on a yellowish-brown hue. This coloration impaired the visibility of the fish when the water was returned to the main tank (Tank 1). To mitigate this issue, samples were specifically taken from Tank 5 just before the water was recirculated to the fish tank. Reducing color intensity at this point is essential for improving water clarity and ensuring better visibility of fish. There is currently no specific regulatory framework that governs aquaponic systems. Nevertheless, the stability and efficiency of aquaponics rely heavily on maintaining appropriate water quality to support the three interconnected components: aquatic animals, plants, and nitrifying microorganisms. As outlined by [PERSON], et al. [30], the conditions include maintaining a pH between 6 and 7, temperatures ranging from 18 to 30 \({}^{\circ}\)C, dissolved oxygen levels between 5 and 8 mg\(\cdot\)L\({}^{-1}\), and nitrate concentrations from 5 to 150 mg\(\cdot\)L\({}^{-1}\)--equivalent to approximately 1.1 to 33.4 mg\(\cdot\)L\({}^{-1}\) of total nitrogen. Beyond these primary parameters, managing the availability of essential nutrients--especially potassium, magnesium, calcium, and iron--presents a significant horticultural challenge in aquaponic systems [31]. The water sample was analyzed for nutrient content (K, Mg, Na, Ca, N, and P), organic matter concentration, and color intensity, as presented in Table 1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Units** & **Value** \\ \hline pH & & 6.5 \(\pm\) 0.3 \\ Apparent color & Scale Pt-Co & 75 \\ DOC & mg\(\cdot\)L\({}^{-1}\) & 39.3 \(\pm\) 0.3 \\ Absorbance (254 nm) & & 0.8326 \(\pm\) 0.0003 \\ Total N & mg\(\cdot\)L\({}^{-1}\) & 151 \(\pm\) 2 \\ N\(-\)NO\({}_{2}\)\({}^{-}\) & mg\(\cdot\)L\({}^{-1}\) & 0.065 \(\pm\) 0.001 \\ BOD\({}_{5}\) & mg\(\cdot\)L\({}^{-1}\) & \textless{DL} \\ Total P & mg\(\cdot\)L\({}^{-1}\) & 13.5 \(\pm\) 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physicochemical properties of aquaculture wastewater.