cant really tell photo quality isnt great. are the leaves turning yellow from the middle to the top of the plant? are the veins on the leaves green yet the rest of the leaf tissue yellow?
its hard to diagnose your problem without information
how old are the plants?
what is your feeding schedule?
whats the dosage of nutrients you are using?
how close are your lights?
are your plants getting enough light?
could be a micro nutrient defficiency like iron if the leaves veins stay a rich dark green while the rest of the leaf yellows and starts to die off