Harry, the growing tips are yellowing indeed, so it may be that. The pots doesn't feel cold at the touch when the light is on, as they're black and it's hot enough in there, but when it's dark they may do (I didn't check) as the temperature goes down substantially.
What do you mean to look at the rootballs? do you want me to snatch them out of the plant pots?
I may be able to sneak some heat mat or something inbetween the pots and switch it on when it's dark... maybe that would help... thanks for the suggestion
The temperature of the nutrient mix is room temperature right now, as I have a big 5L carafe with the bat guano tea, which I'm using to feed the plants now. I'm not using enzymes though... I don't remember why, but I decided not to use them when I was planning everything at the beginning... maybe I should use them?
Cheers
If the temps are going down 'substantially' at night that could be your problem (or a part of it).
I suspect the pots are getting chilled at lights out and that if you carefully turn the pots upside down and lift the pot off the medium you'll see the roots are undeveloped and/or brown. This is causing some lock out/deficiency or another.
Get some warm under the pots. If your heat mat is too small for all the pots, move them around every few hours.
Stop the substantial temp drop at lights out.
Give them some enzyme such as Cannzyme at every watering for a week.