Suggestions here are correct. You don't need to feed seedlings if they're in a good soil. Coco isn't complex enough on its own to provide nutrients. If you transplant to a better medium the plant will bounce back and once it's doing better it will grow exponentially. If you start again I'd probably go with soil as well. Good luck.
Ok. I will try replanting the poor old seedling in soil and see what happens. If it doesn't recover I'll start again. Not the end of the world. Thanks everyone for your advice. I appreciate it.