Help stalk turning black

Need help, I've got 2 plant in 20litre container there on week 3 after being rooted, I recieved them as clones.

I've had them under 300w cfl, which I've just switched to 600 MH, and one of the plants stalk is turning black, this started happening after I topped the plant or I thaught I did, pics are below.

They had 2 cold nights recently temp dropped to 17 16 , but other than that temp has been bang on, I over watered them originally so gave them 1 week to dry and only fed the water since then, now I can see the leaves are going light so the need nutes now which I will give them tmz.

But the plant is question , the pics are from a few days ago and now its turned really dark black and spreading to other parts.

Purpling stems can happen with colder temps. Pics of the entire plant will help.

GBAUTO said:

Purpling stems can happen with colder temps. Pics of the entire plant will help.

Click to expand...

I'll send a fresh new pic tommz., when they wake up , well today, but the purple looks black now

They getting enough magnesium? Whats the pH running?

There getting plenty of calmag, I overfedthem on 1st week, so gave them 7 days to dry, then gave them just water with a ph of 6.0, but prior to that when I overfed I used ph 5.8, but my run off was coming out 5.0, I just gave them some nutes now because as you can se below they showing deficiencies, and run off ph was 5.2, and ec was bloody 3.5, although I made my mix at 1.1, how the bloodyhell do I fix this, I'm growing in coco ecothrive charge, but symptoms are starting to show on both plants now.