Typical reason leaves (lower) turn yellow is nitrogen defiency. In late flower this is normal but ur plants seem too young for that. I would recommend foliar feed of distilled water pH adjusted to 5.8 (hydro) or 6.5 (soil) with nitrogen/bloom fert at .25 TSP/ Gall. Thats the best I can say with the info u gave.. dude I don't know about most but hydro was a rough experience for me. I had 6-10 failed crops using bubble buckets and I almost gave up.... finally switched to soil and now my life is much easier LOL... good luck man don't give up. Educate and pass on your knowledge, free the world.