Yep, PP nailed it.
That's nute burn.
And from the damage pattern, I'd say that you started feeding them too soon.
Seedlings need to be ramped up slowly.

From the new growth though, it looks like they are getting more tolerant.
Cut back on the feeding.
The damaged leaf won't improve, but the new growth will tell you when to feed again.

:twocents:
And if you are using one of those $7 soil-moisture/PH/light meters with the dual probes, throw it out!
They lose any semblance of accuracy as soon as the get dirty, then they lie to ya.
Mo' betta, to just lift the pot.
That give you an idea of how much water they lack.

I keep a small postage meter in the grow room.
If a plant feels light to me, I pop it on the scale and see exactly how light.
If a 16 Oz. beer cup filled with coco gets under 5 Oz. I add 2 or 3 Oz. of water.

You may not need a scale, but when I first used one, I was dismayed at how bad my guesstimates had been.
Used to think I could tell what the RH was too, 'until I got a good RH meter.
I had no clue.

Aloha,
Weezard
Weezard Reviewed by Weezard on . HELP! leaves turning brown Hi i am a first time grower and i am having problems with my plant i am not sure if its from over feeding or what i am using 24-8-16 mairical grow and i feed my plant once every 4 to 5 days i usually water when my gauge is at 1.5 at the bottom of my pot and the top soil is completely dry. it seems like its only the fan leaves that will turn completely brown and over time it works its way back until the leave is pretty much gone.. the other leaves only get it at the tip but it don't go any Rating: 5