So imma offer some advice for ya. I'm watching your grow and to be blunt, it's slow growth... Very slow.
Your tight node spacing says its getting enough light, yet why are the leafs so dark and heavy? I added my picture of tight node spacing, yet my leafs are so thin and very light green. My growth rates blow most people's minds. Im still under a year experience but I don't fuck around lol.
I ran 5 different strains all in the same nutrient solution. The one inch tap root got 225ppm of flora trio with
calmag and
hydroguard. You know what all 5 of my strains had in common during their first two weeks.. All light green stems with no red to them, and all my fan leafs looked like the picture. It looked like my growth doubled daily. In your picture I see bright red stems and dark green leafs. So imma say you need to get off fox farms and get on trio with mag.. . Then run low ppms. Imma say your still overfeeding it... Which is stunting the growth substantially.
Also your res Temps are to low for my liking. I like 72f water with environment Temps 77f. In my head this means the nutrients can flow faster through the plant as they are closer to the temp of the plant. I have tried 68 water temps VS 72 and it's like night and day for me.
So if you can warm up your water by moving your air pump to an area that's not so cold I think it would help but only if you lower your feed. Nothing will help fix that issue in my opinion...