Nitrite (NO2) is an unstable compound, and with enough oxygen will be easily oxidized to NO3 by nitrification bacteria. The concentration of the NO2 tended to increase during the experiment, especially in T1. Although during the experiment NO2 increased, the concentration was still safe for the life of nile tilapia. This is because oxygen supply is enough for NO2 oxidation process. Thus, NO2 did not accumulate in the system. In the circulatory system, NO2 concentration should not exceed 10 mg/L for long periods and in most cases should remain under 1 mg/L (Losordo et al. 1998). On the other hand, NO3 concentration also increased during the experiment, and the concentration at the end is greater than the beginning of the experiment. NO3 is the source of N besides NH4, and these results indicated that romaine lettuce prefers NH4 than NO3 as a source of N. NH4 is a form of N that can be used directly by plants, while NO3 should be converted in advance into another form. According to Xu et al. (1992), in the plant tissue, accumulation of NH4 is higher than NO3, especially when grown under high N conditions. NH4 assimilation occurs relatively rapidly by plants and metabolic reactions are more efficient than NO3. The low NO3 removal by lettuce has been documented in other aquaponic systems (Lennard and Leonard 2006; Buzby and Lin 2014). During the experiment, the concentration of NO3 was still supportive for the life of nile tilapia. According to Watson and Hill (2006), NO3 should be maintained below 100 mg/L.