yelow plants. pleace help

I don't know why, but my plants started to yellowing.
I use hydrophonic sistem. Evereting looks well - temperature 28C. Water PH - around 7(not perfect, but ok, I think). Lamp Metal Halogen 400w. The tops are 35cm /10" from the Lamp. Is the distance ok?
I'm not sure about the humidity. Is that the reason?
Shall I remove the yellow leaves, or the plants need them?
Is it Ok for the plant, cutting leaves
Pleace, advice me somethong, if you know what I have to do!
Thanks!

how old is it?

usualy means lack of nitrogen