Plant growth in hydroponic systems is affected by the nutrient solution concentration. The present study was conducted to assess the effect of nitrogen (N) and potassium (K) contents on the concentration of some nutrients, chemical fruit qualities, and yield in tomatoes (Solanum lycopersicum, cv. Hirad). The experiment was arranged as a completely randomized design with four treatments (T1: N160 + K350 ppm at the vegetative stage and N130 + K250 ppm at the reproductive stage, T2: N170 + K360 ppm at the vegetative stage and N140 + K260 ppm at the reproductive stage, T3: N180 + K370 ppm at the vegetative stage and N150 + K270 ppm at the reproductive stage and T4: N190 + K380 ppm at the vegetative stage and N160 + K280 ppm at the reproductive stage). Results indicated that the increase of N and K supply in nutrient solution decreased the Ca, Na, Mn, and Zn concentrations in the leaves. In contrast, the element content in the fruits was unaffected by the increase of N and K (except for Cu) and the second treatment (T2) had the most beneficial effect on the Cu concentration in the leaves and fruits of tomato. Enhancement of the total soluble solid, chlorophyll, lycopene, and total acid could improve fruit quality in tomato fruits. The best treatment in the present study was N170 + K360 ppm at the vegetative stage and N140 + K260 ppm at the reproductive stage.