First time grow in years

Propably magnesium. If you can foliar spray her for 5 days with epsom salt. 1 tsp for a liter of solution. This will help you to identify if its really a magnesium defficiency before changing her nutes in the water. Also idk if you are watering correctly since these signs could also be because of a root problem, like too much water / too little oxygen, a too small container or bugs lile fungus nets.

I'd say more so magnesium vs potassium. Check out this leaf chart to help pinpoint issues. https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg Good advice about checking out CaMg, just Mg can use epsom salts, as I don't see any issues with Calcium. Good Luck!

Magnesium deficiency. Plants in coco need lots of cal/mag.