## 4 Discussion
### Fish Histopathology  
Histological studies of farmed fish in aquaponics systems are limited. [PERSON] et al. [16] performed the histopathological examination in sea breams in a brackish water aquaponicsystem, while [PERSON] et al. [45] examined the histopathology of sea bass in a freshwater aquaponic system. Nevertheless, the experimental design of these studies did not include nutrient supplementation.  
Fat deposition in the fish liver is affected by the dietary lipid content. Lower dietary lipid content results in lower liver lipid content and smaller and lesser lipid droplets [46]. In the present study, red tilapia were fed ad libidum twice daily with a fish feed containing 6.5% crude fat. Minimal lipid accumulation was detected at the liver of control and Fe treatment and mild lipid accumulation to the Fe + K treatment. No steatosis sign was detected in any of the treatments. Liver histopathology of the Fe + K treatment also showed the presence of granulomas, regions with larger nuclei, necrotic regions, and regions with haemorrhage. These alterations, by the frequency of their appearance in the examined fish, were characterized as mild. Since the liver is an important store of energy reserves, where dissection is possible, the hepatosomatic index (HIS) is often used as an estimate of the energy status of the fish [47; 48]. According to [PERSON] and [PERSON] [49] (2017), a decrease in HIS has a relation to toxicity on fish. Values between 1.3% and 1.97% have been reported for red tilapia (_Oreochromis_ spp.) [50; 51; 52; 53]. Our values of HIS are in agreement with these studies, indicating no toxicity. Midgut had a normal structure in all three treatments. Individual lesions were detected in the midgut in 2 of the 15 examined fish and were characterized as random and non-specific.  
Gills are the main respiratory organ of fish. Changes in dissolved oxygen and water temperature can lead to the rapid and reversible morphological change of the gills. Several species such as _Oncorhynchus mykiss_, _Rutilus rutilus_, _Perca fluviatilis_, _Anguilla anguilla_, _Amblopilites rupestris_, _Microperus salmonides_, and _Carassius carassius_ have been reported to be able to reduce their respiratory area and their oxygen demands as well [54; 55; 56; 57; 58; 59]. Red tilapia is a warm water fish and can tolerate low dissolved oxygen values of about 4 mg/L [37]. The primary lamellae hyperplasia observed at the present study in all treatments may be a physiological result as the dissolved oxygen was between 8.32 and 8.41 mg/L, higher than the tilapia's lower demands. [PERSON] et al. [8] reported that in an aquaponic system, the high level of suspended solids could provoke damage to the gill structure, such as epithelial detachment, hyperplasia, lamella fusion, and reduction in epithelial volume. In the present study, the histopathological examination of the gills revealed similar results as [PERSON] et al. [8] described. Uneaten food and feces were daily siphoned; however, a breakdown in small particles still occurred. These particles are very difficult to collect and are potentially dangerous.  
In general, the very limited cases of mild histopathological alterations in liver, midgut, and gills and, moreover, their complete absence in the majority of fish indicate that tilapias of our study were well adapted to the aquaponics system.