If you're giving them nutrients, stop and wait until they have 3 sets of adult leaves. That could be nute burn. And even when you do start feeding, use 1/4 strength for the first few waterings.

Or your lights could be too close to the plant. Fluorescents avoid this problem.

Or you could have a fungus. But the first 2 are more common.

I always suggest that new indoor growers start by practicing on dirt. You can make a lot of mistakes without killing the plants.

Nice to see another chica growing the shrek.