And you can't have too much heat?I would personally say heat because u can have to much light and seedlings will be stunted because it is to much light. what light are u using for the seeds and how are u heating the seedlings?
I have yet to run into someone who has given their plant too much light.i know u can have to much heat but he provided the heat range so if he keeps it in that rang he is good and now dont give them to much light....