Lack of nutrients or toxicity?

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.

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

I would probably look at a CalMag supplement first. I grew without it for years but the first time I started using coco I began with an assortment of leaf oddities. CalMag cleared them right up.