I'd be looking for some new medium and I always recommend a soilless mix such as Sunshine Mix or ProEarth. These are neutral and won't contribute to what you're seeing.. You can typically find this at an indoor gardening store and it's not very expensive.. These mixes include the proper mix of perlite as well, let alone the fact that you can pretty much rinse the medium with flushing should you run into problems. It would also suit you well to get a small T-5 panel or something similar to use with seedlings and early veg. These run much cooler and you can put them as close as 4-6" without hurting the plants. This will also prevent any unwanted stretching of the plants during early growth. An alternative would be to get a switchable digital ballast that you can turndown.. good during veg and managing heat problems. The ones I use have a 360/400/600/superlumen setting.. Never use superlumen, it shortens the life of the bulbs.

If you go with the new medium, just repot your seedlings being gentle through the process. I'd also drop the nutes for a couple of weeks.. The strong will survive and recover.

The other thing I try to tell folks is to start thinking in weeks instead of days/hours. Developing patience will serve you well when growing..

Good Luck!

Farmer Rich