## 3 Results
### Coagulation-Flocculation in Continuous Mode
#### 3.1.3 Nutrients
In some cases, the relative excess of nitrogen increased following coagulation, as also observed in batch mode [14]. Therefore, phosphorus supplementation is recommended in the aquaculture system under study. Since the primary objective--color removal--was successfully achieved, the N:P ratio should now be adjusted by increasing the current phosphorus concentration. The N:P disproportion creates a high risk for eutrophication and algal bloom formation, especially in systems where phosphorus becomes the limiting nutrient. Although phosphorus removal through coagulation/flocculation is generally considered beneficial in major treatment plants, in this case, the high removal efficiency may be seen as a drawback. The observed discrepancy between phosphorus and total nitrogen removal efficiencies can be attributed to the nature of the compounds involved. Natural coagulants are more effective at removing particulate matter and phosphate ions, which readily form insoluble complexes and flocs [16]. In contrast, nitrogen is often present in more stable, dissolved forms--such as ammonium, nitrate, and organic nitrogen--which are less responsive to coagulation-based removal [52]. Regarding nitrite (NO\({}_{2}\)), concentrations remained relatively stable across all treatments, with only minor variations observed. This outcome is expected, as nitrite removal is primarily carried out by biological processes. However, it is important to confirm that the use of natural coagulants did not affect nitrite levels. Regarding the mineral composition of the treated water, notable changes were observed in the concentrations of aluminum, calcium, iron, and magnesium. Calcium, which plays a vital role in fish osmoregulation [53] and directly contributes to overall water hardness, decreased significantly after treatment--from 71.5 mg:L\({}^{-1}\) in the raw effluent to values ranging between 28.1 and 47.9 mg:L\({}^{-1}\). A similar trend was observed for magnesium, with concentrations falling from 17.0 mg:L\({}^{-1}\) in the untreated water to between 4.1 and 6.7 mg:L\({}^{-1}\) post-treatment. These reductions are noteworthy, as calcium and magnesium are the primary contributors to water hardness, a parameter that influences fish physiology and overall aquaculture performance [54]. Water hardness requirements vary depending on the species cultivated. For instance, low hardness levels (50-100 mg:L\({}^{-1}\) as CaCO\({}_{3}\)) have been associated with reduced feed efficiency and lower stress resistance in _Microterus salmonics_[54], while levels around 400 mg:L\({}^{-1}\) as CaCO\({}_{3}\) are reported to improve stress tolerance and energy efficiency. Based on the initial concentrations of calcium and magnesium, \begin{table}
\begin{tabular}{c c c c c c c} \hline  & \begin{tabular}{c} **Coagulant** \\ **Concentration** (mg:L\({}^{-1}\)) \\ \end{tabular} & **pH** & \
\begin{tabular}{c} **P** \\ **(mg:L\({}^{-1}\))** \\ \end{tabular} & **(mg:L\({}^{-1}\))** &
\begin{tabular}{c} **Ca** \\ **(mg:L\({}^{-1}\))** \\ \end{tabular} & **Mg** \\ \hline Raw & & & & & & \\ effluent & - & 6.5 \(\pm\) 0.3 & 13.5 \(\pm\) 0.2 & 151 \(\pm\) 2 & 71.5 \(\pm\) 0.1 & 17.0 \(\pm\) 0.5 \\ CS & 50 & 6.5 \(\pm\) 0.3 & 1.1 \(\pm\) 0.2 & 126 \(\pm\) 5 & 44.2 \(\pm\) 0.6 & 6.5 \(\pm\) 0.3 \\  & 10 & 8.0 \(\pm\) 0.2 & 1.22 \(\pm\) 0.04 & 97 \(\pm\) 2 & 30.2 \(\pm\) 0.4 & 5.7 \(\pm\) 0.3 \\ Tanfloc & 50 & 6.5 \(\pm\) 0.3 & 1.0 \(\pm\) 0.1 & 106 \(\pm\) 3 & 34.7 \(\pm\) 0.5 & 5.3 \(\pm\) 0.3 \\  & 25 & 7.0 \(\pm\) 0.2 & 1.07 \(\pm\) 0.02 & 93 \(\pm\) 3 & 35.5 \(\pm\) 0.5 & 5.6 \(\pm\) 0.3 \\ Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) & 50 & 6.5 \(\pm\) 0.3 & 1.0 \(\pm\) 0.1 & 123 \(\pm\) 4 & 47.9 \(\pm\) 0.6 & 6.7 \(\pm\) 0.3 \\  & 20 & 8.0 \(\pm\) 0.2 & 1.13 \(\pm\) 0.01 & 89 \(\pm\) 3 & 31.4 \(\pm\) 0.4 & 5.3 \(\pm\) 0.3 \\  & 50 & 6.5 \(\pm\) 0.3 & 1.052 \(\pm\) 0.002 & 123 \(\pm\) 3 & 40.6 \(\pm\) 0.5 & 5.9 \(\pm\) 0.3 \\ FeCl\({}_{3}\) & 25 & 8.0 \(\pm\) 0.2 & 1.22 \(\pm\) 0.02 & 106 \(\pm\) 4 & 28.1 \(\pm\) 0.4 & 4.1 \(\pm\) 0.3 \\ \hline \end{tabular}
\end{table}