The aim of the research work was to evaluate the effects of two levels of electrical conductivity (EC) of the nutrient solution, i.e. high night and low day EC vs continuously low EC, on water consumption, physiological responses, yield, and fruit quality of two tomato cultivars (‘Diana’ and ‘Naomi’). The use of a nutrient solution with high EC during the night (6 dS m-1) did not modify the plant growth. The daily water consumption of tomato plants was not modified by the nutrient solution EC, however water consumption during the night was on average 9-19% of the total daily uptake. Both at the 2nd cluster setting stage and beginning of harvests, the high night and low day EC treatment decreased stomatal conductance (13%) and leaf net CO2 assimilation (8%). The values for ‘Diana’ and ‘Naomi’ of yield of tomato (4.5 and 3.0 kg plant-1, respectively, for ‘Diana’ and ‘Naomi’) or mass of fruits, fruit dry mass, titratable acidity, and ascorbic acid were not influenced by the nutrient solution EC. However, in the cherry tomato type (‘Naomi’) the number of fruits within optimal size values (< 40 mm), which is of crucial importance for their marketability, was higher with high EC in the night hours. The total soluble solids were higher with low/high (day/night) EC than continuously low EC (7.0 vs 6.6 °Brix for ‘Naomi’ and 4.6 vs 4.3 for ‘Diana’), so nutrient solutions with night high and low day EC appear to improve the quality of fruits of soilless grown tomato plants without to modify the fruit yield.