First true set of leaves are yellowing, brown edges & spots? HELP (Coco Coir w/ Critical Kush)

Hello everyone

I have a couple grows under my belt and I'm starting another w/ Critical Kush and I'm having some issues that just started happening overnight it seems & I can't figure out the problem right off the bat. I'm currently 6 days from sprouting. Hoping someone here can help me out, thank you all.

Growing Medium: Coco Coir & Perlite (1:1 Ratio)
Water: 180ppm TAP (dechlorinated & pH'd to 5.
Lights: 600W MH 2+ft away from solo cups.
Area: 4x4x7 Tent
Surface Temp of Seedling: Around 70 - 75°F

*NO nutrients have been used yet
*I have been doing a light watering 1 - 2x a day since I am in coco just to keep medium damp
*I am keeping a dome lid on the solo cup to keep humidity for seedling

- First true set of leaves are yellowing with light brown edges and spots starting to appear. This all just started to happen overnight, before she looked a lot healthier than this, 6 days from sprouting, what could this mean?



Thank you very much for you time & consideration!

try a different grow medium. imo.
everyone has their tips and tricks, no right or wrong way about it.
but i would switch to coconut husk, or clay pebbles. they are reusable, and cleaner imo.

Ph to 5 seems really low, i wouldnt go any lower then 5.5. You might be locking out nitrogen