‘Durinta’ tomatoes were grown hydroponically using rockwool substrate in greenhouses to assess the seasonal and postharvest changes of selected quality attributes including lycopene and total soluble solids (TSS, % Brix) concentrations in fruit when grown under varied electrical conductivity (EC) in the nutrient solution. Two levels of EC examined in this study were 2.4 and 4.8 dS·m−1 for standard low EC and high EC treatments, respectively. All fruits at light-red and red ripeness stages were harvested and weighed every week, and nine fruits visually representing the median group of red fruits were selected from each EC treatment and subject to measurements of lycopene and TSS concentrations. Of 53 harvest weeks (Dec. 2005 to Dec. 2006), 45 weeks were subject to fruit quality analyses at harvest and 3 weeks were subject to postharvest quality analyses. Lycopene concentration and TSS showed seasonal differences with larger variation in lycopene, but the high EC treatment induced an overall average of 18% greater lycopene concentration and a 20% greater TSS. The regression analyses indicated that efflux solution EC (EEC) was the most influential factor for both lycopene and TSS concentrations, but secondary influential factors were greenhouse temperature for lycopene and daily light integral for TSS. Postharvest storage test showed that selected fruit quality attributes (lycopene, TSS, ascorbic acid, and total phenolics) changed minimally or not at all for 10 days when stored at 12 °C, a widely recommended tomato postharvest storage temperature. Overall, we consider that producing lycopene-rich tomato by controlling EC of nutrient solution was feasible during year-round greenhouse production using a high-wire rockwool culture system.