Middle Upper Leaves Yellowing Brown Spots

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?

How often are you feeding? What pH?

cal mg add some prob will fix maybe iron def too