^^ doesn't look like nut burn(hopefully you haven't started adding much if any at all), this early on just try to water less and go from there...good chance their growth will be stunted for a few days, but you should be okay.
also...you said your seedlings "rapidly stretched". If you used the term how I think you did, then this means your light is not close enough to the plants and they are "stretching" to reach the light. If you meant they grew rapidly then disregard this part >.<
Hi Bob, Hydrodan
Thank you for your responses.
Bob: I read different sources and watched videos and I saw recommendations to create a very mild feeding solution.
I made a solution to feed them at about 180 ppm (a low dose of the mix of General Hydroponics Flora Series, flower/gro/micro in equal proportions).
I found today that my PH of the feed was 5.9. I lowered it. So, yes I was concerned it could have been a nute burn. I watered with plain water today.
The plants seemed too tall to me after the first 2 days of growth. I figured, the light was too high. The humidity dome is at 9". As I am using a Gro-Smart Tray, I used risers to lift it up. The plant were closer to the light this time than before. This was one of the reasons I suspected that the damaged tips of the leaves might be a light burn (though I have never seen one in person).
The leaves look like they dried/burned and then rotted a little from water in the humidity dome.
HydroDan: I do not believe they are overwatered. Yesterday when I came to check them they were too dry. I had to dip them in the feeding solution because watering with a dropper was not enough.
Before I left for the night, I watered them and for the first time I left the openings of the humidity dome open. I read I have to harden them before transplanting, so I started to reduce humidity.
This noon when I came to check on them again, the Rapid Rooters were not as dry as yesterday but they were "dry-ish".
Do you have any other ideas as to what it may be? I stopped misting them too.