hey I have/had as it is passing the exact same problem. See pics
I dont know 100% why it happened but. I had mine under a t 400W MH and It was too hot. So i first moved it under some T5's. the plants were still getting worse but at a much slower rate.
I think decided to transplant the seedlings into new soil. I had them in Black gold organic potting soil. It was a little bit too strong. So now I have them in a mix to peat moss, sand and Perlite. They are doing great now.
I would suggest using that mix, its just the cheapest potting mix they have and its great for seedlings as it has such good drainage.