Signs of deficiency starting to appear. Need help

Hi guys first time posting but have been using this forum for a while now to gather usefull info which has helped me out a great deal so thank you. Anyways im starting to see signs of some sort of deficiency in the lower/older fan leaves. Between the veins is starting to yellowing and it's slowly making its way up the plant. I'm also seeing some red leaf stems starting to appear. I have attached a couple of pics below

Setup is as follows

Tent size: 1.5x1.5x1.8 roof cube
Lighting: 1000w Hps
Fan sizes: 5inch extractor, 4inch intake
Temperatures: Light on 25-27oC/// Light off 20-22oC
Current stage: Day 25 of 12/12
Veg time: 4 weeks
Grow medium: Biobizz allmix
Nutrients: Bio Bloom(Bio grow used in veg but stopped using after week 2 of 12/12)
Water source: Tap water coming out @ 7.6ph, bubbled for 24-48 hours before using
Current nutrient dosage: 3ml of bio bloom per litre of water going in @ 6.6ph (nutrients given every watering)

defanning is part of growing....just pick them off.
this will increase air flow thru the bottom of the plant preventing real issues along with promoting growth upwards.
just my 2 cents.

good luck

Yeah. Just yank it man.

The yellowing at the bottom of the plant can be a few different things, all not really a problem.

Mild under-watering: can cause lower fans to yellow, not a problem.

Genetics: Some plants like to cannibalize their lower fans no matter what you do. These
types of plants usually make a nice clean smoke.

Mild nitrogen deficiency: The lower fans are the first to go if the soil is a little bit low EC, not a problem.

Mild magnesium deficiency: The lower fans have interveinal chlorosis (the yellowing inbetween leaf veins). Most indica plants get this when they are growing fast.

Root-bound: When a plant no longer has space to create more roots it will be forced to recycle the lower parts of the plant to make new parts on top.

Senescence: Most plants start to go yellow when it is naturally done with the flowering cycle and it prepares for death. If you let this happen the smoke will be clean tasting with white ash.

From your pics i would bet its just the plant cannibalizing the lower leaves because at this early flowering stage they require a lot of nutrients and its sucking some out the lower leaves to get what it needs.

I think you answered your own question 'deficient'

may need to up the amount of nutrients

something similar w/red stem described here

https://www.icmag.com/ic/showthread.php?t=349872&page=8

what is this strain ?

Your 3-5 Weeks into 12/12, the plant just finished its stretching (ate lots of food) & is now in the flower bulking period (eating lots of food). Just up your N slightly, or overall ppm. Like others have said, let the old large fans eat themselves up & then prune them off. Having good airflow to the inside of the plant, and the top of the growing media, is extremely important.