Help edge of leaf starting to turn brown

Hi, yeah CalMag would be the first thing I would try too, I would also double check the PH of what I was feeding her.

Other than that, from the pictures she looks a little droopy maybe and I see a couple of slightly cupped leaves. That could be a bit of over watering and/or heat stress or maybe just how that stain grows.

but what ever you got going on there, they look like very healthy girls and should have no problem get through this.

If the damage is only on that one leaf and the rest of the seedling looks healthy, then I wouldn't worry about. Keep an eye on things and let us know if any of the upper leaves start looking off. That first set of leaves can go weird sometimes and as long as it doesn't spread I don't worry about it.

Just feed her PH'd water for now and she should be good.