Looks like nute burn from the cheap ass nutes your using. Flush then start using nutes for coco like canna coco or terra.
either nute burn or its just had a very hot day that stresses the shit out of it.
This is a problem going into flower.
You cant be serious with the lighting. Ditch the incandescent light and lower the cfl to about 6 " .
This is why they are so leggy weak and stunted. They look about 3-4 days old not 3-4 weeks.
From my experience you get 100% or close with small rockwool cubes . Keep wet until they surface.
When they have sprouted reduce watering the cubes, feed them 1/4 strength hydro veg nutes increasing as they get bigger.
That's a really dumb way of checking if it needs water,.
You need to let most of the pot dry a lot more than that, when its light to lift then water. As the plant grows faster and outgrows the pot you can water more often. Also when it sready to water ,saturate the pot instead of dribbling...
It is nute burn and the reason for early leaf yellowing/necrosis is nute/salt burn not N def.
Still , this is a very good grow you have here and the weight should be nice.
Really that's a cloning/seedling tent and they have already out grown it. Get a bigger tent they will end up 3x that size with stretch. I like pumpkin soup.
Try to remove them by hand with a bright cfl to check under leaf or use chemicals doesnt really matter, just get them off as they will probably cause the buds to mold and reduce final weight as they increase and get bigger.