I'm only now just learning hydro myself but first, check your roots- are they white or brown?
Cover that other hole in the res! The roots are probably JUST getting to the point now where they are poking out of the medium and you don't want them exposed to light.
They look hungry too. While you are fixing the other stuff, hit them with a foliar feed of a complete grow fert with 1/2 teaspoon per gallon epsom salts added to it if you have any kicking around.