Increasing the amount of nutrient available for each plant will minimize pH and conductivity changes.  In hot weather, insufficient nutrient volume could result in conductivity soaring to toxic levels which could seriously damage your plants.  Nutrient volume should be large enough so that no more than ~10-20%* of its volume is used prior to the next water top-up. 

Water losses through plant leaves in hot weather (above 30 deg C / 86 deg F) can be around 5 litres (1.2 gal) per day for each mature tomato plant and 1 litre (1 qrt) for mature lettuce. Therefore, as a guide, allocate at least 10 litres (2.5 gal) of nutrient solution per tomato plant or around 2 litres (2 qrt) per lettuce and other similar size plants.