your PH is too high for a hydro setup.. you need to reduce your PH nutrient solution with some PH down.. 6.8 ought to do fine, but im told on firm assurance 6.2 is really where its at for hydro.

Hope this helps.. cause this explains a possible simple solution to your problem.

Remember, add a bit.. if it starts to recover.. add a bit more the next day or too.. not too much at once!! NOT TOO MUCH AT ONCE! :-)

I say that twice, because you don't want to seal your plants demise.. hehe

Have fun.

Peace,
Denial