much more info needed to help. organic grow? hydro? what type of nutes are ya using, lights? type of soil?...........I'm assuming it's a soil/ peat grow. most minor problems will start with the bottom leaves. usually a sign of over watering, mediums that are not airy enough will rot the roots at the bottom of the container. Are you making sure you have a good run off when you feed? this gets rid of the salts from the last feeding, which will cause a gradual yellowing from the bottom up. burnt leaves at the top is usually caused by your lights to close to the plant/canopy. over feeding nitrogen would cause browning and burnt leaf tips at all levels of the plant. 1000w lights that are not air cooled need to be 2 feet or more above plants. a little closer if 600w. Unless your into your final 3-4 weeks of flower ( when fan leaves will have some yellowing) my best guess would be too heavy of growing medium and/ or over watering without proper run off. when ya put up pic's, give all the info you can , including : room temps/ humidity, type of soil or grow method, nutrients used, how much ,how often, PH level of water, of nutrient mix and most important the strain your working with. all strains are not equall. some you can blast the shit out of, some like low to medium amounts.........oh yea what size containers are the girls in if this is a soil grow. wait to get the pics up and provide all the needed info