Aquaponics combines hydroponics and recirculating aquaculture elements. Conventional hydroponics requires mineral fertilizers in order to supply the plants with necessary nutrients but the aquaponics systems use the available fish water that is rich in fish waste as nutrients for plant growth.
Principles of Aquaponics
Another advantage of this combination lies in the fact that excess nutrients do not need to be removed through the periodical exchange of enriched fish water with fresh water as practiced in aquaculture systems. The system results in a symbiosis between fish, microorganisms, and plants, and encourages sustainable use of water and nutrients, including their recycling.
Within this synergistic interaction, the respective ecological weaknesses of aquaculture and hydroponics are converted into strengths. This combination substantially minimizes the need for input of nutrients and output of waste, unlike when run as separate systems.
Plants need macronutrients (e.g., C, H, O, N, P, K, Ca, S and Mg) and micronutrients (e.g., Fe, Cl, Mn, B, Zn, Cu, and Mo), which are essential for their growth. Hydroponic solutions contain well-defined proportions of these elements  and are added to the hydroponic solution in ionic form with the exception of C, H, and O, which are available from air and water.
In aquaponics systems, plant nutrient input from the fish tanks contains dissolved nutrient-rich fish waste (gill excretion, urine, and feces), comprising of both soluble and solid organic compounds that are solubilized to ionic form in the water and assimilated by the plants. To sustain adequate plant growth the concentrations of micro-and macronutrients need to be monitored. Periodically some nutrients may need to be added to adjust their concentration, for example, iron is often deficient in fish waste [24,25].
Aquaponic systems need to be able to host different microorganism communities that are involved in fish waste processing and solubilization. Ammonia (NH4+) from fish urine and gill excretion can build up to toxic levels if not removed from the system. This can be done by step-wise microbial conversion to nitrate.
One of the most important microbial components is the nitrifying autotrophic bacteria consortium that is established as a biofilm on solid surfaces within the system and is principally composed of nitroso-bacteria (e.g., Nitrosomonas sp.) and nitro-bacteria (e.g., Nitrospira sp., Nitrobacter sp.). The ammonia within the system is converted into nitrite (NO2−) by nitroso-bacteria, before being transformed into nitrate (NO3−) by the nitro-bacteria .
The final product of this bacterial conversion, nitrate, is considerably less toxic for fish and due to its bioconversion, is the main nitrogen source for plant growth in aquaponics systems [27,28,29]. In most systems, a special biofiltration unit where intensive nitrification occurs is required.
The optimal ratio between fish and plants needs to be identified to get the right balance between fish nutrient production and plant uptake in each system. Rakocy  reports that this could be based on the feeding rate ratio, which is the amount of feed per day per square meter of plant varieties. On this basis, a value between 60 and 100 g day−1 m−2 has been recommended for leafy-greens growing on raft hydroponic systems .
Endut et al.  found an optimum ratio of 15–42 grams of fish feed day−1 m−2 of plant growing with one African catfish (Clarias gariepinus) for eight water spinach plants (Ipomoea aquatica).
Hence, finding the right balance necessitates fundamental knowledge and experiences with regard to the following criteria:
- Types of fish and their food use rate;
- Composition of the fish food, for example, the number of pure proteins converted to Total Ammonia Nitrogen (TAN);
- Frequency of feeding;
- Hydroponic system type and design;
- Types and physiological stages of cultivated plants (leafy greens vs. fruity vegetables);
- Plant sowing density, and
- Chemical composition of the water is influenced by the mineralization rate of fish waste.
Additionally, since fish, microorganisms, and plants are in the same water loop, environmental parameters such as temperature, pH, and mineral concentrations need to be set at a compromise point as close as possible to their respective optimal growth conditions.
As outlined above, the aquaponics system can be seen as the connection between conventional recirculating aquaculture systems (RAS) and hydroponics components. In short, water recirculates in a loop as it flows from the fish tank to filtration units, before it is pumped into the hydroponic beds that are used as water reprocessing units.
The filtration units are composed of mechanical filtration units for solid particles removal (e.g., drum filter or settling tank), and biofilters for nitrification processes (e.g., trickling or moving bed biofilter). Although system configurations and complexity can vary greatly.
Three types of hydroponic beds are commonly used: media-based grow bed, Deep Water Culture (DWC) bed, and Nutrient Film Technique (NFT) gutter-shaped bed. The media-based grow bed is a hydroponic trough filled with the inert substrate (e.g., expanded clay, perlite, pumice, gravel), serving as root support and microbial substrate. The water is commonly supplied in an ebb and flow pattern, ensuring sequential nutrition and aeration.
The DWC system consists of large troughs with perforated floating rafts, where net plant pots are inserted. In the DWC system, these plant pots are generally filled with media, such as Rockwool, coco, or pumice that support the roots, which are then continually submerged in the water tank.
The Nutrient Film Technique (NFT) consists of narrow channels of perforated squared pipes where the roots are partially immersed in a thin layer of streaming water.
With respect to a holistic system approach, there are many ways to frame an aquaponic system in terms of hydrological and functional design. A few scientific papers provide working knowledge about different designs and key parameters.
It is particularly noticeable that DWC systems are mainly used, and important design parameters such as fish to plant ratio or daily feed input are sometimes missing from the literature. It must be mentioned that some costs (i.e., labor costs) are not taken into account, so the financial viability can only be partially estimated.
Apart from the UVI system, there is a lack of scientific literature when it comes to aquaponic experiments on a large scale and during long time sequences. Moreover, many experimental setups published are small-scale replicates of the UVI design. Limited data on the cost and potential profit of such systems are available [24,39,41,42].
As aquaponics is still in a maturing experimental phase, scientific research has focused more on technical aspects than economic viability. However, economic challenges need to be addressed. Experiments covering bigger production systems exist, but they are performed by private research centers or companies, whereby confidential findings are not always made accessible to third parties.
Source: Goddek, S., Delaide, B., Mankasingh, U., Ragnarsdottir, K. V., Jijakli, H., & Thorarinsdottir, R. (2015). Challenges of sustainable and commercial aquaponics. Sustainability, 7(4), 4199-4224.