Plant yellowing

Thats a Mg def, magnesium is not easy for the plant to take from the tap water; especially from the 4th week of flowering the plant start to eat a lot of it, it’s better to supplement a sugar type nuts ( like sweet, bud candy, molasses, ata flavour ecc) that help avoid this purple stems , yellowing and then necrosis leaf from the bottom ( Mg definciency) . With Ro water i feed calmag one time and the next Mag from week 4 , before always CalMag. I’m a soil grower. Hope that helps. ✌️🏽 Sorry for the english

If you check your runoff ph and ec levels when you see problems, that tells you what you need to fix. Diagnostics of weed 101. You become a plant doctor. Cheers

Overall pretty healthy looking. On your 4th week of flowering so some nutrient draining from low peaves is normal. But that is a lot of yellowing for a week. First, if your water is usually 6.5pH, do a couple watering with 6.0pH as it could be nutrient lockout. Or if you havnt been giving them much nutrients at all, then I'd give them more for at least 1 more week. (Try less nitrogen if possible to avoid more clawing) Good luck!