The edges of the leaves are yellow which is weird because that typically means theres too much nutrients in the medium and you say you're not using any. One thing I do notice is that the edges of your leaves are pointing up too much which means your plants need water. I would try to water them a little more first and see how much that helps. Are there any red stems one the plant because that would mean your plants need nutrients bad. Check this plant problem shooter out:

Cannabis Nutrient and Deficiency Table