I'm sure it's K deficiency as it has all the signs of it late stage, waited until got yellowing on sides of leaf. 

I checked my PH as I know I was low last time I checked, 5.9 and now its a comfortable 6.7, I then checked my ppm and it's 700. So with all my results it's potassium ( k ) my mistake but definitely potassium deficiency, not phosphorus

Got in a muddle over p and k 😂 anyways fixing it with some plant magic organic pk boost , should recover , definitely not nute burn 👊

In my defense I was baked while typing 😂