Population of Egypt is gradually increasing and there is a necessity to find out new techniques to reduce the gap between population needs and agricultural production. One of the new techniques called “aquaponics” is which we can utilize the outputs of fish farming in growing vegetables, i.e., lettuce, cucumber, tomato, cabbage and so on. In this technique a minimum requirements of nutrients could be used, furthermore removal the fish feces (Khater, 2006).

Aquaponics is the integration of aquaculture (fish farming) and hydroponics (growing plants without soil). In aquaponic system the fish consume food and excrete waste primarily in the form of ammonia. Bacteria convert the ammonia to nitrite and then to nitrate (Diver, 2000, Bromes, 2002, Rakocy, 2002, Selock, 2003, Lee, 2004, Okimoto, 2004, Karen, 2005, Nelson, 2006a, Nelson, 2006b, Nelson, 2006c, Nelson, 2008, Graber and Junge, 2009).

Aquaponics has several advantages over other recirculating aquaculture systems and hydroponic systems that use inorganic nutrient solutions. The hydroponic component serves as a biofilter, and therefore a separate biofilter is not needed as in other recirculating systems. Aquaponic systems have the only biofilter that generates income, which is obtained from the sale of hydroponic produce such as vegetables, herbs and flowers (Rackocy and Hargreaves, 1993).

Aquaponic system is one of the economical solutions for getting benefits from the water-waste from the fish farms as it save nutrients and produce fresh vegetables. With using this system successively its cost will be decreased and became more economic. The produced plants via this system considered as an organic product which is more safe for human consumption (Khater and Ali, 2015).

Small proportion of ammonia is toxic to fish, when as nitrate is not toxic to fish. If nitrate increased over a specific limit it will be toxic to fish eaters (human being) and cause nitrate pollution and the eaters will suffer from methemoglobinemia disease. The blood of the affected people became brown and will not be able to carry oxygen to the rest of human organs (Tucker and Boyd, 1985). To avoid this problem in aquaculture, part of water should be discharged daily and add fresh water instead. Another solution to this problem is establishing hydroponic system attached to the aquaculture and cultivates plants in the hydroponics in order to save discharged-water and gets use of existing nitrate.

Benefits of aquaponics are conservation of water resources and plant nutrients, intensive production of fish protein and reduced operating costs relative to either system in isolation. Water consumption in integrated systems including tilapia production is less than 1% of the required in pond culture to produce equivalent yields (Rakocy, 2002).

Plant cultivars with high biomass production may have a high potential for being used in integrated water treatment and plant production system. The highly productive hybrid Napier grass cultivar, Pennisetum purpureum × Pennisetum americanum cv. Pakchong₁, may be a candidate species for being used in such systems. We studied the effects of inorganic nitrogen form (NH₄⁺, NH₄NO₃ or NO₃⁻) on growth, morphology, N uptake, water content and mineral allocation in this species under hydroponic conditions at equimolar concentrations (500 μmol N L⁻¹). Generally, the N-form significantly affected growth, biomass allocation and tissue nutrient and mineral composition of the plants. The hybrid Napier grass grew better on NH₄⁺ compared to NO₃⁻, and the plants supplied with NH₄⁺ contained three times more chlorophylls than plants supplied with NO₃⁻ alone or NO₃⁻ combined with NH₄⁺. The morphology of the plants was, however, not affected by N source, except for the shoot to root ratio, which was lower in NH₄⁺-fed plants. The relative water content of the leaves was lowest in the NH₄⁺-fed plants, but the transpiration rate was not affected, indicating that NH₄⁺ nutrition and the associated low tissue concentration of K had negative effects on the water use efficiency of the plants (Jampeetong et al., 2014).

Rana et al. (2011) studied revealed that PO₄-P was removed by 58.14–74.83% with maximum removal at 50% wastewater. More than 75% removal of NO₃-N was observed in all treatments. Both COD and BOD were reclaimed highest at 100% wastewater by 61.38% and 72.03%, respectively. Ammonium-N concentration was subsided below the toxic level in all the treatments. The population of coliform bacteria (Escherichia coli) was reduced to 91.10–92.18% with maximum efficiency at 100% wastewater. Growth performance was observed relatively better at 100% wastewater. Crop production as the value addition of this technology was also recorded maximum at 100% wastewater. The bioaccumulation of Cd and Ni in tomato crop was far below the threshold level, but the bioaccumulation of Lead (Pb) and Crom (Cr) was above the safe level by 80 times and 660 times, respectively. The aquaponically reclaimed water can be reused in agriculture, aquaculture and industries.

Yang et al. (2015) studied a hydroponic system was applied as the final treatment stage of source-separated human urine after urea hydrolysis, induced-struvite precipitation and ammonia stripping in tropical conditions (Singapore). The results showed that water spinach grew efficiently in the pretreated urine with 1:50 dilution ratio at the growth rate 0.68 cm day⁻¹, leaf number 2.27 pieces day⁻¹, shoot dry mass 0.33 g, water content 93.86%, and nitrogen and potassium conversion rate 0.46 and 0.51 mg/mg, respectively. This hydroponic system removed 58–66% chemical oxygen demand (COD), 41–49% total nitrogen (TN) and up to 47% total suspended solid (TSS), indicating sufficient urine stream polishing. Nitrification was observed when COD reduced by 60%, possibly because of oxygen competition between nitrobacteria for nitrification and microbes for COD degradation. The kinetic study revealed that zero-order model provided best fitting for COD and ammonia–nitrogen (NH₄⁺-N) removal, while second-order model was more suitable for TN removal.

Tomato is one of the most important crops worldwide, because tomato is the second most important vegetable in the world after potato, with an annual production of 161.8 million tons in 2012.Tomato is one of the most important economic vegetable crops, practiced by the Egyptian farmers. The total cultivated area of tomato is about 454,800 Faddens and total production of tomato in 2012 was 8.6 million tons (FAO, 2012).

Due to gradually increasing of production costs, it is required to maximize the utilization of available resources. Nutrients in the recycling water is considered one of these resources, therefore, the main objective of this investigation was to study to which extent the content of nutrients in water discharged from fish farms is sufficient for growing tomato plants, in order to reduce the using of chemical fertilizers and increase the water use efficiency, consequently increase the profits of production.