White Dwarf/Onyx/Critical/BFBC - slow progress

Hi all, this is my first AF grow, have grown a couple of blueberry clones but no AFs so i do kind of get whats going on.
First of all, my setup:
DR90, 250w MH, Manrose 4" exhaust fan, passive intake, small circulatory fan, growing in 15l Airpots using coco.


Watering schedule: - using Advanced Nutrients Micro Grow Bloom.


- 4l each every 4/5 days (until a small amount of run-off occurs)
- Feedling 1ml per 4l (1/4 dosage) every other water
- Using tap water corrected to 5.8/6.0pH
- Last feed was 6.0pH & 400ppm


Now i have read that some people suggest not feeding at all until 3/4 weeks in but they started fading in colour, the feed did seem to pick them up some what.
The plants do seem to be growing a bit slowly, the pictures taken are of them at 2 1/2 weeks old. (Onyx being the one without its seed baggie)
I had been using a dr60 so couldn't fit all the airpots in, WD/Onyx/Critical were all in 1l pots until a week ago. - possable cause of stunting?
Wondering if anyone with more knowledge on AF could help me out.
All help is much appretiated, take it easy, Collie

I normally only start feeding mine with nutes at the end of the 3rd week. Maybe move them a bit closer to the light and ease off the nutes. Your substrate could be lacking a few things too - my all-mix soil lacks mag & cal. Yellowing tends to suggest nitrogen, but the slowed/stunted growth is a pH problem. Is it normal for coco to have a lower pH?

manual lockout of nitrogen due to overwatering ?
ph problem ?
did you properly flush the coco before you started ?

ph checks out ?
i know from experience i use coco in my mix that this stuff is super salty and needs a good soak and rinse
but then needs to be allowed to drain and dry up a bit before you plant your seed not really a pop and grow
how did you prepare your medium if you dont mind me asking?