Plants are looking great really... remember, with rapid growth comes need for foods. Looks like you could use more Nitrogen by the looks of the lower leaf colors going a bit pale.
Best advice I got that got rid of that necrosis your getting is this.. begin using Calmag at wk 2 of veg and silica..
I use calmag_+ by Envy. I chose it because it has an NPK of 4-0-0, since then I've had almost no issues..
The silica is something found natural outdoors in the dirt.. by adding it your increasing your plants resistance to stress, building up it's cell walls to help fight bugs.. so many good points to it...
Have you removed your Cotyledon leaf's yet? right about now is a good time, I do it at 2wks old.. remove the little round leaf that comes out 1st and the single blade pot leaf just above it. this will promote upward growth as it allows the plant to concentrate more energy on the upper part of the plant.. works great for me so I do it to all of my plants.. and topping now after trying it this run, I topped 3 of the 4 and they are out performing it big time..