Purple and dying leaves are usually the result from a lockout of nutrients. Some yellowing of fan leaves is normal during flowering, but during vegging, it is a big red flag. A lockout is usually caused by one of two things...one being an incorrect Ph range, and the second is a excess of one nutrient causing another nutrient to become unavailable. A possible 3rd issue is nutrient burn, which I don't see here at all. My suggestion would be to do a Ph and PPM/Ec measurement to see where you are at on Ph matters, then check PPMs (nutrients and minerals). Eith Ph or incorrect ratios can affect P and K absorption, and to my old eyes, it looks like you are being deficient in P or K, probably caused by too much P or K, interfering with the absorption of the other. Purple stems are fairly common, but when they get streaky or run in veins, then a lockout is usually to blame.
FWIW, I grow in
Promix, which is very easy to grow in and very forgiving. It doesn't have any nutrients in it, so the feeding is entirely up to the grower. I use Jacks 20-20-20 fertilizer about every 12-15 dys during strong vegg growth. This has left me without any problems for decades. Foolproof? Almost! I only recommend this as it is VERY easy to work with and provides great results... especially if you are a newer grower and still learning about your plants. Your plants look pretty good other than the problems mentioned, but your problem should be corrected before anything else. I don't know anything about
Biobizz or many other treatments or additives, but plants take a very small amount of nutrition... much less than the maker's recommend. Be careful of the dollar grab, where outrageous prices are attached to common nutrients, simply because the bottle is pretty or the name is catchy. For almost every nutrient out there, there are very cheap versions available. Just looking to your future....