what are you watering with? tapwater? is the veins in the leaf yellow? or is it the light?
In my opinion it looks like a mag deficiency but could be cal or iron too.
Edit: On second thoughts the more i look at it the more i think its a potassium def
I am not familiar with those nutrients. I'm fact, I'm not entirely confident giving advise here.
However, the recent pic you put up looked to be dry. If it feels as dry as it looks you will be watering regardless.
The reason I would give 1/4 strength recommended nutes is because with your medium having no nutes, I just can't imagine it can possibly being a toxicity.
Sorry for the late reply. Got caught up