The seedlings are probably quite over fed. Seedlings generally spend the first few weeks of their growing lives trying to establish roots so they do not need much food at all. I generally do not feed mine anything serious until about the 4th week and even then it is half strength feeding.
When I see them bushed up a little, and in particular, a large proportion of the leaves extend out beyond the edge of the pot, I know the roots have filled out much of the soil area and its ready for some adult food.
The reason I know this is because as a general rule the roots will reach out about 3/4 the length of the fan leaves. So if you look at your seedlings ou can see that the roots will only be filling out about the middle half of the pot. So they are not ready for adult food yet.
Seedlings do not have all the adult parts that clones have and that makes them quite susceptable to root burn.
Best to flush them and keep the feed to the bare minimum until you see the majority of the fans extend out to just over the edge of the pot rim.
I am not saying to not feed them, just saying that those burnt leaves to me look like an indicator of the roots being shocked by adult feeding. What you see above is generally what is happening underneath.
I marked out an area to aim for. When a plant bushes out that far that means the roots have nicely filled out the root area and the plant is ready to pump.
My pants are 3 weeks along. 3 started from seed one is a clone. The clone is fine. My leaves keep crusting up and turning brown and dying. I seem to not be able to figure out what is going on.... here is what im doing..5 gal pots-fox farm ocean for soil-fox farm big grow two table spoons per gal every other feeding-water is 6.5 ph. under a 4' t5 light
