## 1 Introduction  
Aquaponics is a multicultural technique that combines the simultaneous fish and plants culture in RAS systems, integrating aquaculture and hydroponics in a soilless system [1, 2]. Aquaponic systems consist of the fish tank, the hydroponic cultivation tank, the mechanical filter, the biological filter, and the sump. The produced ammonia by the uneaten fish feed, fish feces, and fish urea is converted to usable by the plants' nitrate through oxidization by nitrifying bacteria [3]. Nitrates are absorbed by the plants, and the purified water returns back to the fish tank. Aquaponics is a sustainable solution with a small environmental footprint [4], as it uses less than 10% of the water used in conventional agriculture [2]. Independence from the soil is also an advantage of aquaponics over soil-based agriculture, as aquaponic systems may be located anywhere, with no need for suitable soil presence [5, 6, 7]. Fish, plants, and bacteria coexist in an aquaponic system. The type of the system, the filter size, fish species, fish biomass, number of plants, and plant species must be carefully chosen in order to have a successful production [8]. Tilapia species, carp, perch, and catfish, are the main species that are cultured in aquaponics systems [9, 10, 11], along with some crustacean species such as _Cherax quadricarinatus_ and _Procambarus_ spp. [12, 13]. Among tilapia species, _Oreechromis mossambicus_ is the most widely distributed. It is easy to keep and breed in captivity. The red tilapia (_Oreechromis_ spp.) in aquaculture is a hybrid between \(O\). _mossambicus_ and either \(O\). _niloticus_ or \(O\). _hornorum_[14]. In 2018 Oreochromis spp global aquaculture production was 1.03 million tons [15]. Tilapia is generally tolerant to pH, temperature, and dissolved oxygen changes. Sometimes multispecies cultivation is more efficient in an aquaponic system. Different fish species used in identical aquaponic systems change oxygen levels and, along with the combined feed input, alter plant species growth [16]. Traditional aquaponics systems send nutrient-rich water from the fish tank to the plants and back (single loop). Decoupled aquaponics systems separate the aquaculture and aquaponics units. In these systems, the sludge from the fish tank is digested in the biological waste system, which provides the nutrients for the hydroponic system. The water sent from the RAS to the hydroponic unit is consequently replaced by clean water, which reduces nutrient concentrations and thus improves water quality [17]. Brackish water aquaponics systems are useful in areas where freshwater is limited. Brackish water aquaponics combines the culture of eurylahine fish species, such as sea bream or sea bass, in combination with a wide variety of plant organisms, such as algae or halophytes [18; 19]. Until now, more than 150 species of vegetables, herbs, flowers, and small trees have been successfully used in aquaponics systems, with lettuce, tomato, basil, eggplant, pepper, or spinach to be the most widely used [20; 21; 22]. Since aquaponics is based on the usage of nutrients resulting from fish metabolism, significantly less fertilizers are required for the efficient cultivation of plants compared to classic hydroponics. Fish feed supplies most of the essential nutrients required for optimal plant growth except for calcium (Ca), potassium (K), and iron (Fe), which are usually inadequate and must be supplemented in aquaponic systems [23]. The relevant literature has documented the effects of K and Fe suboptimal availability to aquaponics-cultivated plants mainly in terms of yield reduction. Nevertheless, in a multicultural system, such as aquaponics, where the emphasis is given on the sustainability and biosafety of the products, crop and fish yield should not be the only criterion. Plant functional responses and fish well-being are also important.