Not sure if it's nutrient burn or if it'...

you shouldnt really be growing in soiless / hydro without a PPM meter , that's one of the way to gage if you're overfeeding or underfeeding - dont bother getting an expensive one just buy 2 or 3 cheap ones because they break easily, once you start measuring your ppm you'll be able to tell what's going on. Hope this helps !

Sounds like you've been given every option out here man you should try to post up some more detailed pictures or at least more ...There is a lot of help here you should be able to hopefully save these girls from death ..

Are you using fox farm? That soil is very hot every one I know has problems with it I don’t for some reason. It would be lack of calmag and nitrogen but that’s my guesses

Hey unfortunately without seeing clear pictures of an affected seedling it will be hard to tell what is going on. I can give you a few of the most common issues that kill seedlings. First is going to be overwatering. Most common mistake people make. The seedling needs very little water. Best to start out just misting the grow medium around the plant with a spray bottle rather than pouring water. The second most common cause is burning them with with nutrients. In a quality soil you wont have to feed them until about 3 weeks or so since the soil has plenty of nutrients to feed them. Plain ph adjusted water is all you need. In hydroponics you want to give plain water for at least 3 days then start off with 1/4 strength nutrients for the first week. Another common cause is heat or light burn. Best to start the seedling off under a low wattage cfl daylight bulb. I use a 23w 6500k daylight spectrum bulb.($3) I start it about 8 inches away and move down a little if it gets leggy. The seedlings dont need much light. They will thrive for about 2 weeks under that light. Most likely one or combination of these common issues is what is killing your seedlings. Hope this info gets you past the seedling stage.

Could be some reasons, hard to tell without knowing some Backgrounds .. 1. What kind of grow medium do you use? Looks like soil in the picture? And If Soil,.what kind of Soil? 2. Do you have holes in the bottom of your Pot? Bad drainage is one of the main problems during the first days. 3. Did you gave them nutrients? And if yes, what and how much did you give? 4. Do you maybe could post a picture from 1 of the Ladys?