Red Stems/Some Leaves Seem to be Too Dark

I have 14 plants with 2 1000w HPS bulbs in about a 8x8ish room in 4 & 1/2 gallon soil containers, they are about 10 days of their veg cycle at 18/6. As clones they were pretty neglected, but with some fox farms soil, nutrients, and about a week later they are looking sooooo much better. However I am concerned because the stems on many of the plants are turning red...I understand this to be a nutrient deficiency....which seems odd to me because I am properly adding nutrients (at least I believe I am lol). Also while about 2/3's of the leaves have a nice green color, some are a little dark and droopy still. Should I just give them more time to recover? I am currently using some grow and microblast nutrients right now, I have a flowing one as well but I figured you used that for flowering not vegetative. Any suggestions on how to fix this?

I know its kind of off topic but I have a scuba tank sized C02 tank practically full, I was considering buying a regulator to help with my grow. However I have heard mixed results....any advice would be appreciated

Thanks - Mr. Bojangles

hard to say about the plants without pics, but sounds like there still bouncin back from the neglection....and YES, from what iv read/heard c02 is BIG difference maker in day2day growth