There's various reasons as to why your plant tips may have burned this early . One of the reasons is that you are using nutrients they might be strong still , i would dilute it even more if I were you 1/4 strength . The other things could be potential light burn 1000w output is a lot of your young seedlings , I would find a way to reduce it to 400 watt at the very most for about a week or 2 then kick it back into 1000w . Theres a few reasons for this a) the plant can only absorb so much photon's so even tho your running 1000watts of light the plant isn't able to use 1000w worth of light b) the light is very intense for a young seedling so you could cause light burns if your not careful . So if your able too dim your lights down to 400w or a little below that and bring it up over time . Reduce the amount of nutrients fed as they are very very young and adjust the doses accordingly. Remember that less is more when they are first starting off .