It does look like either not enough light penetration down to those leaves and/or a nitrogen def....and as far as the pythium or root rot goes - I don't believe h202 is a very good option for root rot for a few reasons.
A.) H202 dissolves back into the air within 24-36 hours or something along those lines
B.) It kills any and all living organisms it comes into contact with, including beneficials that line the outer layer of your root mass that help nutrient uptake and protect roots from nasties.
C.) I'm pretty sure too much h202 can harm roots, too an extent at least, but this info could be off.
I was using h202 when I caught root rot, applied more, just slowed it down, and finally I ready this page
http://www.growweedeasy.com/root-rot I decided to begin using Hydroguard. Literally within 3 days of my first application, there was an explosion of pearly white roots out of the older infected ones, and even the brown ones became much less brown/infected looking. Now, there are likely a 1000 additives that has the same effect, such as Great White, maybe (MAYBE) SM-90, Voodoo Juice, etc etc....because when beneficials outnumber nasties like pythium, the pythium can't breed and multiply as easily if at all. Ever since I started adding Hydroguard to my RDWC I haven't had the first problem with it...anywho that's my .02, not saying I'm right or that I'm wrong, it's just my experience. Happy Growing and Happy Holidays, my friend!
*EDIT* I believe achieving a sterile root zone isn't nearly as effective as having a root zone full of beneficial microbes/enzymes when it comes to DWC/RDWC, but that's just MY opinion and is what has worked best for me...doesn't mean that it's the best method or better method at all.