N deficiency or deficiencies @ first week of flowering 12/12 help!

Hi,

How do I fix this issue(s)? The leaves are starting to yellow at an alarming rate but the top shoots are nice and healthy though. Is it natural for the plant to go through a deficiency transitioning into flowering? sorry first time doing an indoor grow with the nice bells and whistles...

Strain: Monster Cookies
Grow Medium: Promix
Using RO water
PH 5.8-6.0
Using house and garden line (COCO A+B)
Co2 Generator set to 750ppm (will go up in ppm weekly)
AC set to 70 degrees
Dehumidifier set to 50%

I don't run co2 but I understand you want temps in lo 90s when using it. With RO you need to add back cal/mag, very important to nutrient transport. I see some clawed tips and curved leaf structure indicating over feeding has taken place earlier in your grow and you may have just watered them or are overwatering. Deficiencies that start at the bottom and work up are mobile nutrient issues (NPK or mag).

So, research optimum temp ranges for co2 supplementation. Get some cal/mag going and I would for sure do an Epsom salt spray for immediate magnesium. Check your nutrient ppm levels and keep track of them, it will help with future grows. Good luck!