Novice tent growers in need of advice

[JKash]

Thanks a lot for the input, much appreciated. Do you have any suggestions for alternative soil? I've ordered a hygrometer and pH tester, hope that will shed some light on the problem.
The feed I've been using is canna bio vega for the veg stage and canna bio bloom booster for flowering phase however I hadn't put any on these seedlings yet, because I thought you had to wait a couple of weeks so it didn't overload the plant - is that right or should I have been feeding because of the coco?
Thanks again!

So with using coco (not experienced user) you may need to water 2 or 3x a day and feed with those waterings as coco does not retain nutes or water for long periods of time i.e. like soil does... so your seedlings were probably underwater and had very little no food available... as to any soil even sterile soil like promix.... there is just enough nutes for a seedling to feed for its 1st 30 to 45 days of life