Hey Kanna, it is hard to tell without pictures in natural light but I would guess it isn't potassium as that normally starts at the bottom with the older leaves and works it's way up. If it is isolated to the top of the plant it could be light burn but that doesn't normally cause spotting then turning to rust. That sounds more like calcium but normally calcium def doesn't show around leaf edges, it tends to be sporadic in the leaf. Sometimes you see magnesium and calcium def together and magnesium affects leaf edges causing them to yellow first then start dying off. If you have some you can foliar feed with that will help out quickly.
Best of luck brother!