Are they all the same strain? Do they come from the same mom? If you have different strains, they might have different needs. To me, it just looks like tip burn. If your other two are not getting the tip burn, then lowering your EC could slow their growth. I would separate the non-performing plant and clean up the dead leaves and branches. Yeah, throw that one in some soil and nurse it along, then rock and roll with your hydro duo. Stay steady with your EC until you see some growth. With your girls two weeks in flower, you might wanna crank up the P and K, but don't push it too hard, they are just starting out.