Tips of my older leaves going white

Growing in Coco coir + Perlite, and the tip of the older leaves are going white, all the new growth is fine.
I'm thinking it's a cal-mag problem ..... as this plant missed out on getting any.
I've added some cal-mag to this mornings feed, so hopefully the newer leaves won't suffer.
Does plants grown in coco need cal-mag with every feed?

That's not a Ca or Mg issue it's pretty classic K def.

You're in very competent hands. Heed the advice. Better buds to all.