Bubba Kush / Exodus cheese / Jack Herer

Just switched timer from 18/6 to 12/12. Topped all three 2 weeks ago. Maybe someone could answer something for me though? Dont know if its to do with the spectrum of light from my LEDs or a calcium deficiency but keep getting a couple of yellow leaves around the bottom of plants, but has never affected any of my grows so just carried on, also growing the same strains but out in the greenhouse in the garden and i dont get the yellowing which makes me believe something to do with the spectrum from light?