Any help with my girls would be greatly appreciated

I have been trolling the forums looking for an answer on this, and maybe I'm just not calling yellow what all the info sheets and pictures posted are calling "yellow". So I'm a little stumped and would greatly appreciate any help those more knowledgeable than I can provide. This is my second grow and never had this issue on the first one. If this is a redundant post and someone else has answered this I apologize. If you know someone has posted an answer, I'd love to get the link.

I have 6 Master Kush (feminized, very bushy!)
Germinated around 5/6 so 3 weeks into veg
Growing in DWC INDOOR with one main resevoir, each girl has her own bucket
Feeding Jungle Juice, currently at about 1/3 strength (I don't want to start a whole pissing contest on which nutrient system is better please)
Also add Cal-Mag and Aqua Shield to the resevoir each time I empty/fill it
I keep ph between 5 and 6
Temp is consistently 79-80F and humidity runs around 40% (I know it's lower than some like but I had no issues with this in my first grow)
Girls are about 8-10 inches tall and will be growing into the screen soon.

I have older-growth leaves turning light brownish between the veins and some drying up. I plucked one off for a good picture. I SUSPECT nitrogen defficiency but being my second grow I just don't know. And all of the information and PICTURES I can find all show/call out YELLOW as the symptom but these are turning a light brown color, much like tree leaves in the autumn. There is no visible discoloration in the veins, stems or underside of the leaves (in fact looking at the bottom you'd never know there was something wrong other than being dried up.

Since the girls are close to the screen and thus hopefully a few weeks tops from flipping the lights I really want to get this cleared up first if at all possible. MOst of these leaves will be removed when the screen fills out anyway, but they are obviously a symptom showing a problem that needs dealt with.

So, can you help??

if you started seeds on the 5/6 of this month surley there only 1 week into veg, as they have spent time as seedlings 2 weeksish (this explains why 8-10inches tall)

did you grow a diff strain before it could be simply that this strain doesn't like the 40% humidity.

do you keep you groom clean, could this be a fungus.

is there good air circ between the leafs under the screen or are they all squished together? they might be stifled.

i would Flush with plain tap water no ph adjusting, for 3 days (new res each day) to make sure girls are clean and hungry, then restart nutes low dose and continue!

Wow! Fast reply Lemon King, thanks!

1. Started seeds 5/6, so whatever terminology, they've been growing since that date.
2. Strain before was different, yes. Weird part is that all the "dry" leaves are on the INSIDE, all facing the resevoir, where the WATER is.
3. I do keep my room clean. It's actually my utility room in the basement. No food, drink, nuthin' but the grow area, AC, my server and a storage cabinet for nutes, etc.
4. I have three small fans in there right now all below the screen blowing into the plants and one 4" high velocity inline fan with carbon filter pulling out exhaust. One of those fans is an intake for fresh air.

I just refilled yesterday with fresh nutrients so may well try a full flush tomorrow if no one else has any suggestions. I certainly appreciate your advice but I also hope someone will look at the picture (beyond all odds) and say "YES, it's XYZ and do ABC to remedy it!"

Wistful thinking

Well I was hoping for more responses but with no other info to go by I flushed my girls last night and gave them nothing but water, ph'd to 5.0. They'll raise that pretty quickly, they always seem to. I go through a lot of ph Down solution. I'll keep this up a few days, with fresh water each time, and hope this clears it all up. Time will tell!

Thought I would upload a few pictures of my girls. The first shows some of the yellowing/browning/rusting/whatever on the leave, while the second shows them in the last few days after bouncing back like...well...weeds! I know the angles are a little funny, but that's where I felt I could safely place the cam without having to move it too often so I can put a timelapse vid together with the images later on.

before

after