Whitered leaves

Probably over or underwatering, after you do your normal watering procedure check the soil for dry spots where the water may not have spread into, or before watering check for moisture from the underside of the pot and check if the soil dried up properly. And water untill you get run off. It may also be from heat or the early stages of K(and possibly P too) toxicity, check the tips of the leaves for burn. But if there is no burn starting up then its most likely a watering or heat issue, if this is happening mostly on the upper sections then its most likely heat, from the color of your picture i assume that you got a HPS/HID flower light, wich makes a lot of heat and would explain it, make sure ur termometer is sitting at the level of the tops of your plants to get a accurate reading, preferably right under the light, in the center of the tent where the heat concentrates the most.