help me. thinking about ph troubles

[iliveinholland]

I have a 7 plant hydroponic system containing circa 25 lt. of nute solution (water + nute). 3 of these had some N deficences but i fixed them. Now i have this plant right in the center that haves some severe problems. I am thinking that could be the ph that cuts off the nutes. here's a pic. I don't know what to do.. pls. Also in this moment i've finished ph papers to measure it. I have to wait a week to have them back. Is there another method to calculate ph?
Pleaselet me know your impressions..

 

[GroHi]

Damn dude, that plant looks fried. You say it is right in the center? Likely candidate is heat burn... the bulb is directly over it, probably w/o air circulation there, and the plants fried from the heat. Seeing that it appears worse up top & still green below supports this... though the plants to either side should also show some sign, I would think...?

Other than that, anything killing off your new growth that is nute/soil related will likely be your micro's, like zinc. I doubt this however & would go w the heat first. Peace.