Middle Upper Leaves Yellowing Brown Spots

[blinken]

Hi all! Bit of a strange problem with this plant. I fed it 50% lucas formula not too long ago. It seems healthy, but now these upper leaves are yellowing and developing brown spots. I know it can't be nitrogen, or iron def. The stems are quite red, so I was wondering, could it be magnesium?

I'm using promix HP as my medium. I was under the impression Lucas contained enough Mg, so I'm not sure what this is. Any ideas?

 

[Mikell]

How often are you feeding? What pH?

 

[B RABB]

cal mg add some prob will fix maybe iron def too