i sugest you fllush the pot out on that middle one, far to many nutes in there i think. need a bit more info on what happened to it, like did the lellowing start at the bottom of the plant and work its way up the plant ? what colour did they turn first, did they show any signs of the leaf tips turning yellow and start dying from the tips of the leaves towards the stem that holds them to the plant????
closer pix would be a great help too