Whats this?

Think you need more magnesium. Most of the cal mags don't have enough magnesium.

 

I would agree with either a little more cal-mag or could possibly be a little nute burn.

 

Discoloration of leaf is very common issue with any leaves, not just hydroponic grow. I was searching internet but didn’t find any promising answer related to this to topic. As far as I can say I do agree with Johnny, maybe it’s Magnesium.

It feels really bad to have malfunctioned leave, through the effort was all good.