It's unfortunate that you lost one but it looks like the rest of them are hanging in there.
Are you able to measure temperature? Humidity and temperature reading might help. I suspect maybe it's a little warm in their environment and maybe the humidity is low. In this regard adding nutrients won't help them, it would only stress them more.
Once they get out of that seedling stage they will be more hardy, you're doing a good job and I know you're going to have success with these seedlings!