## 2 Material and Methods
### Experimental Design, Fish Rearing, and Plant Growth Conditions  
In total, 270 red tilapias (_Oreochromis_ spp.) were left to acclimatize in the experimental units for 15 days before the commencement of the experiment. Subsequently, the fish weight and length were measured, and fish were homogeneously distributed to the fish tanks in order to avoid statistically significant differences in initial weight and length among aquaponic systems. A total of 30 red tilapias were placed in each fish tank with an average weight of 8.74 \(\pm\) 0.023 g and an average length of 8.30 \(\pm\) 0.001 cm. The resulting average breeding density per fish tank was 655 g/m\({}^{3}\). All experimental procedures were conducted according to the guidelines of the EU Directive 2010/63/EU regarding the protection of animals used for scientific purposes and were applied by FELASA accredited scientists (functions A-D). The experimental protocol was approved by the Ethics Committee and conducted at the registered experimental facility (EL-43 BIO/exp-01) of the Laboratory of Aquaculture, Department of Ichthyology and Aquatic Environment, University of Thessaly (n. 329309/24-11-2020).  
Fish were fed twice daily ad libitum with a commercial floating pellet diet (47.5% protein and 6.5% crude fat). Feeding was performed by hand. Fish tanks were cleaned daily by siphoning, and uneaten food was removed. Food consumption per fish tank was calculated by the difference between the amount fed and the amount of uneaten feed collected (corrected for leaching losses).  
Rocket plants (_Eruca veisaria_) were grown from seeds in an unheated greenhouse until the 6-true-leaf stage. A total of 108 rocket seedlings were chosen, showing no statistically significant differences in their morphometric characteristics (height, number of leaves). Twelve rocket plants were evenly placed in each hydroponic tank, 20 cm apart from each other. Plant positions were carefully selected to ensure the homogeneity of the light environment; each plant was exposed to 300-450 umol m\({}^{-2}\) s\({}^{-1}\) of photosynthetically active radiation (PAR). The artificial light was supplied by a 600 W HPS lamp (SYLVANIA, 230 V) placed 65 cm above each growing area. The photoperiod (10 h light: 14 h dark) was controlled by a timer.  
The Fe and K supplementation in the water of the aquaponics systems resulted in three treatments (three replicates per treatment):  
(i) Control group, no nutrient addition.  
(ii) Fe group, only Fe-DTPA was added (Fe target concentration 5 mg/L).  
(iii) Fe + K group, Fe-DTPA and K\({}_{2}\)SO\({}_{4}\) were added (target concentration 5 mg/L and 120 mg/L for Fe and K, respectively).  
Fe and K target concentrations were selected according to [PERSON] et al. [25] and [PERSON] et al. [26] for leafy vegetables hydroponic cultivation. The photometric determination of Fe and K concentrations in the inlet point of the hydroponic cultivation tank was performed twice a week (Hach DR3900) and was followed by the necessary nutrient supplementation to ensure that the targeted Fe and K content was maintained in the recirculating water. The required amount of nutrients was dissolved in 4 L of chlorine-free water before added to the system. Ca foliar application (1 mL/m\({}^{2}\)) was performed twice a week in all groups to avoid Ca nutritional deficiencies.