Answer
You need to monitor oxygen saturation of water, since plant roots need to do aerobic respiration, and the pH of the water to be sure that nutrients are available, as well as nutrient concentrations to be sure that they are sufficient for lettuce production.
Work Step by Step
Look through the chapter to see the role of oxygen in plant, especially root health, and things about soils which support plant growth. Here, the hydroponic solution takes the place of the soil and should have those same properties.