I suggest you move your light closer........major stretching starting and the one is praying for more light.
What is your medium and why didn't you start in a smaller container?
I've yet to see a successful grow with coco and dry amendments.........maybe this will be the first.
Looks like a calcium deficiency.
Good luck and welcome to RIU.
Feed at least once per day to substantial run off.........no need to weigh the pot.........that's soil stuff.
Never let the coco dry and never use plain water.
I would flush your coco with a mild nute solution, then start feeding daily to run off.