Nothing to submit, although i seen to have a brown edge on one of my plants, could anyone diagnose or guide me as to what the problem could be

PICTURE OF DAMAGED PLANT BELOW. Note the brown spot..??? any advice.?
BalloonBoy Reviewed by BalloonBoy on . My Grow Diary G13 Twenty Pot Wilma System Ive started my Diary on day 10. I brought the seedlings on in a propergator after 24 hours shoots were showing, after 3 days they were 2" tall.. Im using PH 5.8 airated water and 1ml per L root stimulant to feed and mist the plants. My conditions are as follows Seedlings in heated propergator 3" Rockwell cubes, sat on Ph balanced Mapito Rating: 5