## 3 Results
### Coagulation-Flocculation in Continuous Mode
#### 3.1.3 Nutrients  
In recirculating aquaculture systems (RASs), careful monitoring of phosphorus and nitrogen compounds is critical, as these nutrients directly influence fish health, plant development, and microbial dynamics within the system. This study specifically focuses on assessing the impact of these parameters--particularly nitrites--on the water quality of the fish tank based on the analyzed samples. Nitrites are of particular concern due to their high toxicity, even at low concentrations [35]. One of the fundamental requirements for the reuse of water in aquaculture is the implementation of nitrification processes [46], which ensure the continuous conversion of ammonium and nitrite to nitrate. The elimination of nitrite is essential, as its accumulation poses a serious threat to aquatic organisms, and in RASs, this is typically achieved through biofiltration units, such as the biofilter represented in Tank 3 (Figure 1). However, it remains crucial to verify whether nitrite is still present in the final treated water. If residual nitrite is present, it can diffuse across the gill membranes and enter the fish's bloodstream, where it oxidizes hemoglobin to methemoglobin--a form that is unable to transport oxygen. This condition, known as \"brown blood disease,\" can lead to fish suffocation, even when dissolved oxygen levels in the water are otherwise adequate [47]. Nitrite levels after water treatment remained within the range of the initial concentration, and none of the coagulants caused an increase in this parameter. Aluminum and iron concentrations were also assessed to determine whether chemical coagulants influenced the presence of these metals in the treated water. Aluminum was detected only in samples treated with aluminum sulfate and Tanfloc, with concentrations ranging from 0.25 to 0.36 mg\(\cdot\)L\({}^{-1}\). While these values are within acceptable limits for short-term exposure, elevated aluminum levels can have adverse effects on recirculating systems [48; 49]. Iron plays a vital role in various physiological processes in both fish and plants, serving as a key cofactor in enzymatic pathways such as chlorophyll biosynthesis, DNA synthesis, and nitrogen reduction [50]. According to [PERSON], et al. [50], iron supplementation significantly improved the growth of _Pangasianodon hypophthalmus_, increasing from 6.32 g (without supplementation) to 7.66 g after 15 days and from 9.3 g to 12.3 g after 90 days with exposure to 1.5 mg\(\cdot\)L\({}^{-1}\) of iron. Although iron leaching into treated water can be a concern, in this case, its presence from an iron-based coagulant is not considered problematic and may even offer biological benefits. In addition to these analyses, the results for phosphorus (P), total nitrogen (N), calcium (Ca), and magnesium (Mg) are presented in Table 3.  
As shown in Table 3, all coagulants tested were effective in removing more than 90% of the initial phosphorus concentration from the aquaculture effluent, demonstrating strong performance in phosphate reduction regardless of coagulant type. In contrast, the removal of total nitrogen was less pronounced. Although the observed reductions were low, this is an interesting result because of the common assumption that natural coagulants, such as tannin-based compounds, are less effective in nitrogen reduction--often attributed to the incorporation of quaternary nitrogen into their structure via the Mannich reaction during synthesis [16]--this study observed a measurable decrease in nitrogen levels even at higher coagulant dosages (e.g., 50 mg\(\cdot\)L\({}^{-1}\)). Nonetheless, the extent of nitrogen removal was insufficient to approach the classic Redfield ratio of N:P = 16:1 (molar ratio), which is widely recognized as a stoichiometric benchmark for balanced nutrient availability for primary producers in aquatic ecosystems [51]. Maintaining the N:P ratio is crucial to prevent nutrient imbalances that can destabilize aquatic environments. In this study, nitrogen levels remained in excess both before and after treatment.