Seems like stagnant air and cold stress to me.
Use a fan to circulate the air around your main stem. By the way never plant this low, air cant flow well over your soil.
Change your light to hps if you can, warms up and glows more.
Znd for your soil, did you wash and buffer your coco? If not flush well and use calmag during grow.
And as everyone said dont overwater! And what is your water source? Tap? If so use a bubbler to aerate and get rid of Cl for some hours before feeding to plant.