over watered, too much heat or both. , cut back on the amount of water you give it, are you home much? if yes, get a spray bottle and put some water in it, then just spray the leaves untill its time to water the soil again.
but first of all, stop watering so much. let the soil dry a little b4 watering again, unless your home a lot, then use a mister to spray them. but first of all. what light are you useing, it looks orange, is it a sodium light?
how close are the plants to the light?
wattage?
how offten do you water them?
we need more info in order to give you the best answers, as many problems can be over or under feeding certain things.
so tell us as much info on your grow op as you can.