Please, please do not buy General Hydroponic products again. They have sold out (and I do mean sold out) to Scott's. Scott's, Miracle Grow, and Monsanto are all in bed together. Monsanto is slowly and systematically poisoning the earth and the people on it and don't care as long as they continue to make a buck. GMO foods are basically created to counteract the effects of Roundup, which poisons the ground.
Do not water until they need to be watered. Being too wet is as bad as being too dry.
Why are you measuring them? The height of the plant tells you nothing about the health/well being of the plant. In fact, when they are stretched, it is more an indication of not healthy.