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