When insufficient nitrogen is available to support new growth, plants can redirect nitrogen from older leaves toward the new growth. If the lower, older leaves in your plants are turning yellow while the newer growth looks fine, add some nitrogen-specific fertilizer such as ammonium nitrate or ammonium sulfate for a quick fix. For a longer term problem, examine your nutrient formula and either make some adjustments, or switch to a different commercial fertilizer (but only one designed specifically for hydroponics!)
Other possible causes for yellowing leaves include pH that is too high or too low, overwatering, nutrient solution is too warm or too cold, or there's too much salt in the water. Start by monitoring your pH regularly - daily and each time you add fertilizer to your nutrient water. It's hard to imagine how you can overwater plants in hydroponics, but it's important for your plants to be able to absorb plenty of oxygen through their roots. If you're using deep water culture, ensure you're maintaining high levels of DO (dissolved oxygen). If you're using ebb and flow or nutrient film systems, make sure your plants have adequate time or a portion of their roots out of the water.