Help! What is happening to my seedling?

Possibly light burn if it started in the center of leaves and moved outward, move it around if it's under a hot spot on your light that could cause it. Lots of other reasons but if it's the only 1 out of 7 and nothing else has been done differently than I'd guess it's lighting.

if your reusing soil this will occur! it seems as if your soil mix is to hot (overloaded with nutrients). Id say transplant as carefully as possible in some Ocean forrest potting soil or even coco loco potting soil from fox farm Good Luck and don't feed and nutes for about 3 weeks afterwards.

That's a nutrient burn, the substrate and/or your water are too "hot." In the first weeks, seedlings can't handle any nutrients. So work with unfertilized seedling soil and only water with pure, pH-adjusted water. Good luck!

Roberts said, "I would guess your ph might be wrong in substrate" Hmm, that is something I didn't check. I checked the PPM in the water and it was 198 PPM in water pail, and 175 PPM in the run off when I was watering all these.

I would guess your ph might be wrong in substrate

Weird. Some seedling just don’t make it man.. it’ll probably die off or grow super slow. I’d watch it for 2-3 days and see if anything gets better but my guess is she’s a gonner

She overdried ??? Your grow medium even have a gap to pot ... Went too dry and burned roots and is dieing now ??? My guess... Good luck !