Irregular rust spots is almost always Calcium deficiency. If it is more consistently the margins and tips with regular spotting (consistent pattern in veins) then more likely Potassium.
Always start with basics:
1. plants cant take any nutrients if PH is off - depending on your medium but...
I've been running my last few years of indoor grows in Coco coir/perlite using GenHydro mineral feed with great results.
BUT a lot of my circle has been raving about Foop! recently so my next grow will be 100% organic!
Grower's Stuff
Medium
Mother Earth 'Coco Bale' & Epsoma Organic...