Its ok to water while the light are on but many people find that smaller pots watered while the lights are on and the fans can evaporate quickly. Just don't overwater. Its hard to keep track of how much ferts are in there when the water evaporates in 4 hours. Try dif things. really i don't think the watering schedule if all else is in check should make much effect on finished product anyway. i once tried soil and just watered whenever and had great results