Thanks, I'm off to the Hydro store right now.
At first, I too thought it was spider mites. However, after looking through your pics very closely, I see no webbing. I also noticed that the brown starts on the outside. So, I think you have a nute lockout or ph problem. I would flush, (preferably with sledge hammer or something similar) and when you feed, give them some cal/mag or Epsom salt (just a little).
Flip the leaves upside down where there is damage, if there are little bugs then you have spider mites. If not then I think its a deficiency as PeaceFarmer notes and yes the necrosis is only on the non green parts of the plants. You could just use plain water to flush as well.